It was the mt was the middle of July, the temperature was 98 degrees, and the thick, muggy air was a dangerous Code Red. It was the kind of day that guarantees a drop in church attendance. And yet on that morning, as on every other summer Sunday this year, an unusually large crowd gathered to worship in All Saints' Episcopal Church in Chevy Chase, Md., a leafy suburb of Washington, D.C. A genial, festive group, the worshipers filled the nave, spilling over into the side pews usually reserved for holidays.

"It's been like Christmas every Sunday," said one delighted Episcopalian. "God bless the Presbyterians."

The Presbyterians who crowded into All Saints' this summer were not new converts but old neighbors, whose Chevy Chase Presbyterian Church was undergoing extensive renovations. All Saints' invited Chevy Chase to use its facilities for the three months of construction work.

But what began as a simple proposal to share space quickly grew into a highly successful series of joint services which had "free tradition" Presbyterians gamely following the Book of Common Prayer, and traditional Episcopalians singing out "Praise Triune God," during the Doxology.

Episcopal and Presbyterian services were held at 10 a.m. on alternate Sundays, with the Episcopal clergy preaching at Presbyterian services, and the Presbyterian clergy preaching at the Episcopal services.

"It was a very easy relationship, a very, very enriching one," said the Rev. Phillip C. Cato, the All Saints' priest-in-charge. "I think everything went so well because no one was asked to become someone else." And yet, he added, each church whole-heartedly participated in the other's service during the alternating Sundays. "People in both congregations have entered into this experience completely," he said.

The Rev. Arthur E. Sundstrom, the senior pastor of Chevy Chase Presbyterian, agreed. "We came from two different worshiping communities, and we respected those differences," he said. "But we have learned a great deal about each other, and most of all, we've learned that what unites us is far greater and more powerful than any differences in liturgical practice."

All Saints' parishioners said they were touched to see their Presbyterian neighbors searching the prayer book for the correct prayer, exchanging the Peace, lining up for communion, sharing the chalice.

"The Presbyterians were very enthusiastic," said Wes Bruner, an All Saints' parishioner who directs the church ushers. "They seemed to approach our services with more enthusiasm than the Episcopalians. Of course, we knew what's going to happen and they didn't. It was new to them."

For their part, Presbyterians said they were moved by the welcoming friendliness of the Episcopalians.

The two handsome stone churches, which have been neighbors for more than 75 years, both sit along Chevy Chase Circle, in a cosmopolitan community that is home to ambassadors, government officials, lawyers and professors.

David P. Notley, a Presbyterian, said he will remember the services at All Saints' with great affection. "It was nice to go across the circle and find we had brothers and sisters over there," he said. "It was very moving to see how two churches expressed the same truth in different ways."

Co-mingling the clergy and congregations from two different churches required detailed planning touching every aspect of worship, yet they accomplished this feat in a few weeks.

"The clergy here have very warm ecumenical relationships," explained Fr. Cato. "When Art mentioned late in the spring that his congregation might be worshiping in their parlor this summer because of repairs, I invited them to use our church. Then I thought, summer is usually a slow time for both churches, so why not hold joint services? Well, we went to work and held the first service in mid- June."

While the invitation to worship at All Saints' was neighborly, it was also a bit unconventional, because the Presbyterians and Episcopalians do not have mutually recognized ministries.

The idea of joint services, which was heartily approved by the All Saints' vestry, was greeted with "delight" by the Bishop of Washington, the Rt. Rev. Ronald H. Haines.

Mr. Sundstrom said his worship committee, made up of church lay persons, recommended the proposal to the session, the Presbyterian body similar to an Anglican vestry.

"The only real bump in the road was the Eucharist," he said. "While both churches welcome any baptized person to communion, our clergy could not celebrate communion in the Episcopal Church. But Dr. Cato very graciously suggested we participate after the consecration, handing out wafers or passing the cup."

To prepare their congregations, both clergymen wrote short essays about their respective services, which were mailed to all church members as a pamphlet, "Ways of Worship." Finally, before the first joint service, Fr. Cato went to Chevy Chase Presbyterian for a question and answer session.

"The questions," he recalled, "were about what to do at Episcopal services, not about theology."

To reassure the Presbyterians, during the joint service period every Episcopal program indicated when to sit, stand and kneel: "Collect for Purity (stand); Psalm 26:1-8 (sit)."

Mr. Sundstrom said Presbyterians, who normally drink grape juice at communion and usually celebrate that sacrament "about 12 times a year or less," were fascinated to find the Eucharist celebrated at each of the Episcopal services.

In fact, when asked what struck them the most about the Episcopal service, compared to the Presbyterian, many pointed to the different eucharistic traditions.

"It's what people are used to, and I'm not being critical, but communion three or four times a year is enough for me," said Samuel Keker, who has been going to Chevy Chase Presbyterian for 50 years. "But I thought the services were beautifully done. I found them very rewarding."

The joint services combined choirs, prayer lists, ushers and altar flowers, while alternating organists and the responsibility for coffee hour. All Saints' was in charge of child care.

Programs were printed by the church responsible for the service that Sunday, but included inserts from the other congregation. Both churches used their own envelopes for offerings. The Presbyterians insisted All Saints' keep all undesignated offerings.

During the last joint service on August 29, in a brief exchange brimming with good will and good humor, Mr. Sundstrom presented Fr. Cato with a stone from the wall of the Chevy Chase Presbyterian sanctuary, as a symbol of the "rock solid" friendship between the two churches. The single stone was also, he said, a symbol of the "oneness" of the two worshiping communities which shared "one common foundation ... our rock and our redeemer, Jesus Christ."

Fr. Cato told the Presbyterians that the Episcopalians had received "far more than we have given. You have been a blessing."

All Saints' recently broke ground for an ambitious building project of its own, and Mr. Sundstrom told the Episcopalians, "We may well be seeing you across the circle next summer. And you'll be very, very welcome." o

Barbara Basler is a member of All Saints' Church, Chevy Chase, Md.