Their nerves still jangling from the collapse of the savings and loan industry, some members of Congress yesterday began pokingaround the foundations of another giant financial structure to see if it, too, might someday fall on their heads.

The structure is the nation's system of "government sponsored enterprises," or GSEs, a group of federally chartered corporations that help provide credit to housing, agriculture and higher education. Together, these operations have borrowed -- or backed others who have borrowed -- to the tune of more than $860 billion, and their debt is viewed by the credit markets as having the implicit backing of the Treasury.

The Treasury is uncomfortable with its good name being used by organizations essentially outside of its control. So recently, it proposed that the GSEs begin to build up what amounts to a more generous cash reserve to protect themselves -- and the taxpayers -- from another costly rescue. The plan is hotly opposed by the GSEs, which fired their opening salvos yesterday at a hearing of a House Banking subcommittee.

This has quickly become a classic Washington confrontation in which all sides have excellent arguments and suspect motives, and in which special interest constituencies are apt play a crucial role.

The GSEs rightfully argue that for millions of Americans they lower the cost of buying a house or paying for college, at no cost -- and with practically no risk -- to the government.

But the Treasury worries that the GSEs pose a significant budgetary risk. Most GSEs are now private corporations that distribute profits to shareholders and healthy paychecks to executives. As Treasury officials see it, the GSEs want all the benefits of their association with the government while having government assume much of the risks associated with their business.

Under the Treasury proposal, each GSE would be required to improve its financial standing sufficiently to win triple-A ratings from two national credit rating agencies. The department would impose its own business plan on any GSE that failed to obtain, or subsequently lost, a triple-A rating.

In addition, the value of the government's financial support -- the risk the government bears -- would be quantified and put on the budget.

Yesterday, two of the biggest GSEs, the Federal Home Loan Mortgage Corp. (Freddie Mac) and the Federal National Mortgage Association (Fannie Mae), told the House subcommittee that the Treasury is going far beyond what is necessary.

Fannie Mae Vice Chairman James A. Johnson called the triple-A rating requirement "a totally counterproductive and self-defeating proposition," and termed the whole plan "not responsible." Freddie Mac Chairman Leland C. Brendsel called the triple-A rating "a high and extreme standard." Both men insisted that independent studies prove their organizations are safe and sound, and that they have never cost the taxpayers a dime.

But speaking for the Treasury, Undersecretary Robert R. Glauber recalled that Fannie Mae ran into trouble in the 1980s and another GSE, the farm credit system, had to be bailed out in the late '80s. The risks, he said, cannot be ignored.

The panel, particularly Chairman Henry B. Gonzalez (D-Tex.), expressed sympathy for Fannie Mae and Freddie Mac and worried that tightening up on them might make it even harder for low-income people to buy homes.

The only tough question at yesterday's hearing came from Rep. Gerald D. Kleczka (D-Wis.), who chided Johnson about the pay of Fannie Mae Chairman David O. Maxwell, who Kleczka said makes "five times what the president makes."

And Glauber may have scored a point when he suggested that even if shareholders -- rather than homeowners -- were forced to bear the costs necessary for Fannie Mae to win a triple-A rating, the company would still enjoy a rate of return higher than 20 percent.

Still, the GSEs have broad support among members of Congress, who enjoy nothing more than providing a benefit to constituents without any costs -- or at least without visible costs. Putting those risks on the federal budget would make those costs all too visible.