Brazil is paving its highways, preening its beaches and cleaning its slums for the thousands of ecologists, scores of Indian tribes and more than 100 heads of state expected in Rio during a U.N.-sponsored Earth Summit in June.

But Brazilian officials have devoted their most feverish recent efforts to ensuring the presence of the one man without whom the U.N. Conference on Environment and Development can't succeed: George Bush.President Fernando Collor de Mello returned empty-handed from two U.S. visits last year after personally urging President Bush to attend. During a visit to Brazil that ended Feb. 10, William Reilly, administrator of the Environmental Protection Agency, was pressed for reassurances that the Brazilians' worst fear won't come to pass - that Mr. Bush, mired in his re- election campaign and skeptical about the summit's prospects, will send Dan

Quayle in his place.

"We really want this conference to succeed," Mr. Reilly said he told the Brazilians. "But (the president) will make that decision down the road a bit."

A main reason for Mr. Bush's coyness is concern that developing countries, led by China and India, will press the United States and other wealthy nations for large and "unrealistic" funding transfers to the Third World to pay for environmental programs, Mr. Reilly said.

In two years of preparatory meetings, negotiators have been debating how to resolve environmental crises such as global warming, forest destruction and ocean contamination. The two-week Rio summit will give world leaders a chance to talk face-to-face and perhaps sign several treaties.

So there will doubtless be plenty of rhetoric. How much joint ecological action will result remains in doubt. The U.N.'s Canadian-born secretary general for the conference, Maurice Strong, has spared no hyperbole.

"This will be one of the most important summits ever held - it concerns nothing less than the fate of the world," Mr. Strong said last year.

In a forested glade alongside the convention hall, tribal leaders from all over North and South America, including the American Sioux, are to gather to press for indigenous rights. Nearly 500 international environmental groups have reserved exhibit space in a bayside park for an ecological fair. In Sao Paulo, hundreds of private industries from around the world will hawk biodegradable wares in an ecological trade fair.

The Brazilian and Rio de Janeiro state and municipal governments are funding a frenzied building and cleanup drive, constructing a new freeway to the international airport; overhauling the Rio convention center; cleaning up 56 miles of beaches hugging the city; and improving sanitation facilities in several slums near the convention center.

All the energy and money underline the decision by President Collor, 42, to use the conference as a principal vehicle for projecting his vision of Brazil as an almost-developed country ready for the 21st Century. Mr. Collor has ordered that his entire government - all 12 ministries and seven secretariats - move operations from Brasilia to Rio during the conference. All the hoopla means a lot is at risk for Brazil.

"It could be a tremendous fiasco for Brazil, because the planning hasn't been that good," said David Fleischer, a political scientist at the University of Brasilia.

Brazilian officials are upbeat. But they, too, believe the chances of failure are high if President Bush decides to snub the conference.

"They think Bush's presence would be critical to a substantive outcome, and we agree," said Steve Schwartzman, a Brazil expert at the Environmental Defense Fund in Washington.