AS IT CASTS about for ways to cut the costs of the fast-growing Medicaid program, the administration has come up with the idea of allowing states to try to get adult children to chip in more for the nursing care needed by their destitute parents. It would certainly be good if all children were willing and able to support their needy, aged parents. But the administration's plan, unfortunately, is likely to produce far more trouble than it is worth.

Most people think of Medicaid as a program for only the chronically poor. In fact, a very large part of Medicaid's costs cover elderly or severely disabled people who have used up all their resources in paying for medical care. If you've had to pay a hospital or nursing home bill lately, you know that it doesn't take long for institutionalized people to run through all their money. The number of very old has been rising rapidly, and with it the demand for long-term care. Roughly half of long-term care is now paid for by Medicaid.

States pay about 45 percent of the Medicaid bill, and some of them argue they could provide better care for the truly destitute if they could make better-off people pay more for the care of their elderly parents. That sounds fair, but the complications are endless. What if, as is often the case, a "child" is also getting on in years and lives on a fixed income? Or lives out of state? Or is a nonworking wife whose husband has no interest in supporting his aging mother-in-law? What kind of complicated rules would be needed to ensure that all children shared fairly in the burden, taking account of the different demands placed upon each? So difficult are the potential problems that the Department of Health of Human Services estimated last year that almost three-quarters of the savings anticipated from the rule would be eaten up in administrative costs.

It is fair to say that Medicaid's creators never imagined that a major part of the program's constituency would be middle-class people. They were quite clear in the enabling statute, however, that contributions--except from a spouse or parent-- were not to be a condition of eligibility. The administration argues that the new regulation is legal since patients wouldn't lose Medicaid if children couldn't be made to pay up. That's a debating point that courts may or may not find persuasive. But it sidesteps the much more important question of who the society thinks ought to bear the burden of caring for those who outlive their own resources.

In the old days that burden often fell to some hapless family member who devoted her life to the care of mad old Aunt Ella. Or families were forced to make difficult choices between sending the kids to college and getting Grandpa the nursing care he needed. Allowing that burden to be shared by all taxpayers, as Medicaid coverage does, has been an enormous relief to many families. Before the rules are altered, Congress should have the chance to think hard about the consequences.