Beneath Canadian satisfaction with President Bush's promise to act on acid rain, there runs a bitter current that says, "It's about time."

There also is considerable reluctance to get too complacent until specifics of Bush's program are known.On Bush's first trip outside the United States since his inauguration, he promised Friday to move quickly with congressional legislation to control acid rain.

He also said after his meetings with Prime Minister Brian Mulroney that the United States would move toward discussions with Canada for an accord on the controversial issue.

"The president's comments on the subject mark a small step forward in the longstanding environmental dispute between the two countries," The Globe and Mail newspaper wrote Saturday.

"However, without an exact U.S. timetable for the discussions, or commitments on the amount or timing of any cuts on transborder air pollution, Canada may still have to wait years before there is tangible relief on the issue," the Toronto daily wrote in front-page coverage of the visit.

Canadian concern about the acid rain, blamed for killing some 14,000 lakes in the eastern part of the country and defoliating millions of acres of forests, has been a dominant issue in relations with the United States for the past decade.

One of Bush's two official visits to Canada as vice president was in January 1987, when he came to Ottawa to hear Mulroney's complaints that the United States was holding back on acid rain cleanup.

Frustration grew in Canada during the eight years of the Reagan administration, when the pomp of summit meetings produced virtually no results on the issue.

For example, the Reagan administration did not once propose legislation to strengthen the U.S. federal Clean Air Act.

More than half the acid rain in eastern Canada comes from the United States, and in some areas U.S. emissions of sulphur dioxide and other materials cause 70 percent of acid fallout, according to the Canadian government.

The program to control Canadian emissions takes full effect in 1994.

"I think Mr. Bush's approach is a 180-degree reversal in U.S. policy," said Michael Perley of the Canadian Coalition on Acid Rain lobbying group.

He told reporters that while Reagan did not even recognize acid rain as a problem, the situation now is that "we've got the right principle, the right approach."

Perley also said the specific amount of emissions to be cut and the timetable for doing so are critical.

Senior Canadian diplomatic officials, speaking on condition of anonymity, said they understand Bush's need to outline the specifics to Congress first.

At a joint news conference with Bush, Mulroney said the president's position "represents quite substantial progress. You know, it wasn't so long ago that Canada was sort of going it alone in many ways in this area."

Bush said he assured Mulroney "that the time for just pure study was over and that we've now approached the time for legislative action."

The United States apparently would be willing to start talks for the bilateral agreement Canada long has sought on acid rain after the legislation is moving forward, possibly as early as this year.

Environmental concerns ran strong among Canadian voters polled during the campaign last fall that returned Mulroney's Progressive Conservative Party for a second consecutive majority government.

Indeed, Barbara Bush was reminded by a reporter as she toured the National Gallery of Canada that lakes portrayed in the paintings she was seeing were endangered by acid rain.