HOUSTON, JULY 8 -- On the eve of their annual economic summit, representatives of the major industrialized nations indicated their desire to show support for Soviet President Mikhail Gorbachev's reform efforts, despite clear differences over providing direct assistance to the beleaguered Soviet economy.

The Soviet aid question is expected to dominate discussions of the three-day summit that begins here Monday. But a U.S. official, describing the differences among the participating seven nations, said draft language for a final statement was "pretty vague."

U.S. national security adviser Brent Scowcroft said that while the summit's likely "primary message will be . . . encouragement" to Gorbachev for his reform efforts, the leaders also will sound "a note of caution" that genuine economic reform is needed before Western assistance can be productive.

The communique was being worked on tonight by aides as President Bush welcomed arriving summit leaders at an indoor rodeo and barbecue in the Astrodome. One draft expressed support for Gorbachev's efforts to reform the Soviet economy, and reaffirmed the willingness of the Western nations "to widen and deepen the scope of cooperation with the Soviet Union."

Also today, President Bush and Canadian Prime Minister Brian Mulroney agreed to begin negotiations shortly on an acid-rain agreement between the two countries. A senior Canadian official said an agreement, which Canada has wanted for years, is important for U.S.-Canada relations as well as the environment.

Bush, backed by British Prime Minister Margaret Thatcher and Japanese Prime Minister Toshiki Kaifu, opposes the plan of West German Chancellor Helmut Kohl and French President Francois Mitterrand to give Moscow $15 billion to $20 billion in direct aid. Kohl is eager to aid the Soviets to offset their nervousness over German unification.

Bush and his supporters said the Soviets had not instituted the kind of reforms of the centrally run Soviet economy that would allow the money to be used wisely. While the administration favors technical assistance for the Soviets, it is balking at sending direct U.S. aid as long as the Soviets spend a quarter of their budget on their military and send $5 billion annually to subsidize Cuba.

"There will not be one monolithic policy from the Houston summit," Treasury Secretary Nicholas F. Brady said today on ABC's "Business World."

But there is a clear desire among all the participants to show support for Gorbachev's efforts.

Scowcroft, appearing today on CNN's "Newsmaker Sunday," said, "I think that the primary message {from the summit} will be one of encouragement for the process that is taking place within the Soviet Union, encouragement for all the things that Gorbachev has done and is proposing to do, but a note of caution that much needs to be done in order to make, for example, Western assistance productive."

Mulroney, on ABC's "This Week With David Brinkley," said, "Political and military signals were sent off from London a few days ago at {the} NATO {summit} and my expectation is that we're going to be considering, among other things, an economic response not only to his concerns but to our own."

On the same program, Secretary of State James A. Baker III added that "What we were able to accomplish in London at the NATO summit will be very meaningful in terms of helping perestroika {restructuring} succeed and maybe more meaningful than providing some $15 billion or $20 billion to an economy the size of the Soviet economy before there is economic reform in place."

Baker said Moscow's military spending and foreign aid are reasons why the Soviets "haven't been able to move forward in terms of {providing} consumer goods and that sort of thing."

Mulroney raised the possibility of a Soviet Union without Gorbachev. If Gorbachev was "chucked out" of office, Mulroney said, "we would be in an infinitely worse position than we are today. And so there is a response that has to come from Canada and from all of the industrialized nations. . . . Collectively, there has to be a significant response."

But he echoed Bush's view that individual countries have to decide for themselves how to respond to Gorbachev's request for assistance to help the crumbling Soviet economy and bolster his sagging political fortunes at home.

Mulroney emphasized that Canada is supplying previously promised farm and commercial credits to Moscow, which the United States does not.

Asked if the Canadian credits undercut U.S. policy, Bush replied today, "Nothing undercuts our policy."

White House press secretary Marlin Fitzwater said today that Gorbachev, in a letter to Bush that was received July 4, asked the summit participants "to consider providing technical economic assistance and credit that could be helpful to his economic reform efforts."

The announcement on acid rain followed a private meeting today between Bush and Mulroney. This issue has been a longtime irritant between the two countries, with Canada pressing the United States to reduce emissions from midwestern power plants that have been spoiling its lakes and forests.

White House officials said that with Congress close to passing a Clean Air Act, they were prepared to start negotiations to reduce sulfur dioxide and other components of acid rain.

Environmental activists here played down the significance of the agreement. "There's less there than meets the eye, and it took 10 years to get it," said Daniel F. Becker of the Sierra Club.

James Tripp of the Environmental Defense Fund added, "They are merely ratifying what they had already agreed to do."

The environment is one of three principal issues at the summit, with the United States resisting European and Canadian calls for swift action to combat global warming. The United States and Canada are also at odds with Europe over ending agricultural subsidies.

Bush began his day by attending church with his wife, Barbara; Thatcher, and Baker and Brady and their wives. Later, the president took a 30-minute jog in Memorial Park where he was joined by several dozen citizens out for runs.