Early this morning, while Margaret Thatcher of Britain, Pierre Elliott Trudeau of Canada and the other leaders he is meeting with here rested in their hotels and villas, President Carter slipped away for breakfast with 12 Benedictine monks.

He shared milk, bread and cheese with the monks in their monastery on San Giorgio Island and later he could hardly contain his enthusiasm in telling some of the others of the adventure they had missed. According to White House officials, the monks had invited the leaders of all the industrialized democracies meeting here for an economic summit conference to the monastery for breakfast. But only Carter, a habitual early riser who has always stressed the importance of religious values, took them up on the offer.

The visit was unannounced and became known only when the president returned to the Hotel Cipriani and, in the presence of reporters, told Trudeau all about it just before a private meeting between the two men.

Trudeau's reaction, and later that of Thatcher when told of the early morning excursion, appeared to be mild bewilderment.

"I had breakfast at the monastery," Carter said.

"Did you have a nice breakfast?" Trudeau inquired.

"Yes, we had milk and bread and cheese," the president said.

At this, Trudeau's lip curled as Carter explained how each of the monks had described his job at the monastery during the breakfast.

"I didn't know they could talk," Trudeau said in a reference to the vow of silence taken by members of some orders of Roman Catholic monks.

"At the meal they let them talk and at special occasions," Carter said.

"They ordinarily talk?" Trudeau inquired.

"Ordinarily they do," the president said. "They have prayers before breakfast and they had a special prayer for me. I told them I needed it."

After the Trudeau meeting, it was Thatcher's turn as she arrived at the Cipriani for her private meeting with Carter. The british prime minister seemed uncertain of how to reply as reporters listened to this dialogue:

"Had breakfast with the monks," Carter said.

"Oh, did you?" Thatcher replied.

"They're really wonderful . . .," Carter added.

"Yes," Thatcher said.

"There are some who do special work on Gregorian chants . . .," Carter said.

Thatcher replied again with "Yes."

Not for nothing is the president's Secret Service code name "deacon."