Peter Barash, his wife, two daughters and mother-in-law were in their Rockville home Wednesday morning, preparing to leave for Dulles Airport where they were to board a TWA flight for a week-long trip to Paris.

"We were all packed . . . . I turned on the radio and heard the report about the TWA plane being bombed [over Greece on Wednesday]," Barash said yesterday from his office as staff director of a House government operations subcommittee. "My wife and I looked at each other. We thought, 'We're going into a war zone.' "

The Barashes decided to unpack, forfeiting the $75 deposits they had paid for each of the five family members who were to go on the trip. Instead, they will travel out West this summer.

People in the international travel industry believe the same decision was reached during the last two days in thousands of other homes and offices across the United States. As a result, travel agents and tour operators predicted yesterday that the aftershock of the Athens bombing and other acts of terrorism in recent months will cause a precipitous drop in the number of Americans traveling to Europe and particularly to such Mediterranean countries as Greece and Italy.

Industry sources said it will take about three weeks to accurately gauge of how many American tourists are likely to cancel plans for vacations abroad. But most seemed to agree with Ray Kane, an executive with Pisa Brothers Inc., a New York travel agency, who said: "If I were a tour operator specializing in Greece or Italy packages, I'd start cutting my overhead."

Dan Bohan, president of Washington-based Omega World Travel, which annually books airplane travel for about 250,000 persons through its 32 offices, added:

"I think this latest incident will prove to be the straw that broke the camel's back in terms of people's concern about their safety. I predict that the market will go down 20 to 25 percent from last year in terms of travel to western Europe in general and as much as 50 percent to Mediterranean countries like Greece and Italy that are perceived as having lax security. You can add another 5 to 10 per cent for every new incident that might happen."

Bohan said that his firm's group tour department had received six calls yesterday morning from groups thinking of canceling their tour plans. "Each of those calls represents about 100 people, and normally we get only one or two such calls over four to six weeks," he noted.

"Historically, a terrorist incident causes an immediate reaction in terms of travel," said Robert Witley, executive vice president of the U.S. Tour Operators Association. "That was the case after the attacks at the Rome and Vienna aiports in December. You get some cancellations and a slowing of bookings over the ensuing 30 days or so.

"In the last three or four weeks, bookings had picked up again," Witley added. "As a result, until Wednesday's incident, we figured we might be able to end the year only 25 percent down from 1985, when almost 7 million Americans traveled to Europe. Now the damage is likely to be a lot worse, especially in southern Europe and the Mediterranean."

Underscoring these gloomy assessments were calls by various organizations yesterday for travelers to ponder the possibility that western Europe has not seen the last of the terrorist incidents that have afflicted the continent in the past year, including the Wednesday bombing that killed four Americans.

The Dallas-based International Airline Passengers Association warned its 30,000 members: "Travel for Americans into the Mediterranean area, unless absolutely essential, should be avoided." And the Air Line Pilots Association said it will ask a London meeting next week of pilots from 60 nations to boycott countries "that harbor terrorists or foster terrorism."

State Department spokesman Bernard Kalb said yesterday that the government did not plan any new advisories about overseas travel, but he noted that "travel is never without risk, and the Department of State always recommends the exercise of prudence and caution when traveling abroad."

Representatives of those countries likely to suffer sharp drops in their tourism revenues sought to assuage the concerns of potential American visitors, noting that the vast majority of tourists in Europe last year enjoyed their vacations without incident.

Dan Martin, consultant to the European Travel Commission, which represents 28 countries, said: "I don't think this will stop Americans from exercising their right to travel freely. Otherwise, it will mean that terrorists and their supporters like [Libyan leader Muammar] Qaddafi will have won because Americans let them influence their life styles."

A more cautious view came from George Kouras, acting head of the Greek government's tourism office in New York. He acknowledged that the latest incident could blunt considerably Greece's efforts to repair the damage done to its tourist industry last summer when the State Department cited the Athens airport for unsafe security after the hijacking of a TWA plane to Beirut.

The result of that episode was 30 percent less tourism business from America than expected. In an effort to lure the Americans back, Greece has embarked on a $3 million television advertising campaign featuring 37 prominent Americans of different ethnic backgrounds talking enthusiastically about "going home to Greece this summer."

"Definitely any event like Wednesday is going to have a negative effect," Kouras said. "By some estimates, the number of American visitors could fall by 50 percent. It's too early to tell the extent, but there's no question that it will be very negative."

Opinions were divided on whether Mexico, Canada and Caribbean nations will reap a windfall from the tourists that Europe loses. But there were signs that many will decide to see America first.

The American Automobile Association, which has 470,000 members in the Washington area, said its local office is receiving 1,000 calls a day, requiring agents to work double shifts to fill requests. And Omega's Bohan said: "I think it's going to be a very big year for Disney World, King's Dominion and theme parks all over the country."