The next fashion flash you hear will not be from Paris, Milan or New York. In fact, they are just about the only design centers that haven't got the word that what makes good sense for a man to wear in hot weather is a loose-fitting shirt. Certainly not a tie and jacket.

In most parts of the world, save Europe and mainland U.S.A., shirts and trousers are the hot-weather male gear for most occasions. In some countries, such as Barbados, the traditional style, even for a senator, is a short-sleeve shirt jacket and matching pants. In Hawaii, funeral notices often read, "Aloha shirts requested."

Earlier this week, when Rep. James Maddox (D-Tex.) appeared on the House floor minus coat and tie, Speaker Thomas O'Neill (D-Mass.) rapped Maddox" knuckles verbally and ordered the congressman to leave the floor. "Perhaps the chair reflects the views of his own generation," said O'Neill. Whatever, his response to Maddox was applauded by his colleagues.

Some of Washington's swanky restaurants, cooperating with the president's order to keep thermostats at 78 degrees, have relaxed the usual de rigueur coat and tie rule. At the Jockey Club, Paul de Lisle will make exceptions for some guests who arrive without ties, but rarely seats those without jackets. George Torchio, maitre d" at Maison Blanche, has had no problems with his jacket/tie ruling yet. For him, too, no tie is more acceptable than no coat.

Several years ago a West Indian ambassador arrived at a posh French restaurant in town wearing an embroidered overshirt and was ushered to the check room for a borrowed jacket and tie. He was eventually rescued from having to cover up his national dress by a persuasive host. That situation is unlikely to arise again, say most of the local restaurants.

Sen. Spark Matsunaga (D-Hawaii) declared yesterday Aloha Wednesday. (In Hawaii, Aloha Friday means Aloha shirts acceptable everywhere. Only not in the courts.) Several of the senator's aides appeared for work in the colorful shirts they usually reserve here for weekends and parties. ("I feel two ways better," said William Baldwin. "More comfortable and more at home." Jerry Comcowich and Sidney Rosen admitted they got envious comments from their colleagues but were looked on as tourists by others.)

But the senator passed up his own declaration. "I think the dignity and decorum of the Senate needs to be maintained," said Matsunaga, returning from a vote on the floor. "A person is so much guided by the manner in which others are dressed. When visitors see members of Congress without jackets and ties it could give a bad impression," he said. CAPTION: Pictures 1 through 5, Counterwise from top right: Rep. James Mattox; Wendell Kellman and Charles Hinds at the Embassy of Barbados in Carib suits; aides to Sen. Spark Matsunaga, Jerry Comcowich, William Baldwin and Sidney Rosen, in aloha shirts; Jose Louis "Pepe" Montesinos, owner of Enriqueta's, in a Mexican guayabera; Ricardo Marasigar from the Philippine Embassy in a silk barong. Mattox by AP; Hawaii photo by Linda Wheeler; others by Fred Sweets - The Washington Post.