As the summit's seven heads of government sat down last night to a sumptuous dinner of salmon and veal in lavender honey sauce, they bewildered their aides by launching into a spirited discussion of international problems posed by the proliferation of drug trafficking.

In what White House spokesman Larry Speakes called an "unexpected twist" in their first group encounter, the seven leaders spoke for nearly an hour about the urgent need to tame the blight of drug addiction in all of their countries.

The catalyst happened to be Nancy Reagan's antidrug campaign, which she is pursuing in Rome while her husband participates in the seven-nation economic summit here. The first order of business among the industrialized world's leaders, even before soup was served, evolved out of Reagan's offhand comments about his wife's trip.

The first lady drew praise for her crusade, which has enlisted the help of numerous spouses of foreign leaders, including the wives of Prime Minister Brian Mulroney of Canada and Bettino Craxi of Italy. Mrs. Reagan's recent summit of first ladies in Washington publicized the common threat that industrialized as well as developing countries now face from widespread drug abuse.

British Prime Minister Margaret Thatcher, echoed by other leaders, said it was now all the more imperative to strengthen international cooperation in fighting the drug trade.

It was noted that harvests of opium and coca, as well as the seizure by police of their addictive derivatives heroin and cocaine, have reached record levels.

Reagan, happily surprised by the intense interest shown in the subject, remarked that "never have I seen all my summit partners so united and determined on a single subject."

Before tackling their normal agenda of trade and political issues, the seven leaders decided to instruct their foreign ministers to hold a special discussion today to find ways of enhancing cross-border cooperation among drug enforcement agencies.

"We have picked up another assignment, which was unexpected," Secretary of State George P. Shultz told reporters today. "It wasn't on our agenda, particularly. It emerged from the discussion by the heads of state on this subject, and it may very well be that one of the most important people to this meeting is not here -- namely, Nancy Reagan." SOME WHITE HOUSE reporters assigned to the economic summit have been snared in a Catch-22 situation since it opened. They were compelled to surrender their passports to the White House for safekeeping before receiving their credentials, but West German police have refused to allow admittance to summit events unless passports were shown along with credentials.

The White House tried to explain to the Germans that they were supposed to honor their own press credentials. But the Bonn police force insisted that the terrorist threat looming over the summit has made it doubly conscious of checking identities. The White House finally gave up and returned the passports to the reporters. U.S. ASSISTANT Secretary of State for European Affairs Richard Burt, who yesterday angrily stalked out of a briefing he was giving reporters after hearing some snickering, made a successful comeback today.

Burt returned to the podium of the White House briefing room and opened his presentation with the deadpan quip, "As I was saying . . ."

Despite the incident, which was the source of much amusement in West German newspapers about the relationship between U.S. officials and reporters, Burt is expected to be nominated as ambassador to West Germany after the present envoy, Arthur Burns, retires this month.