Welfare benefits nationally dropped an average of 29 percent, after adjustment for inflation, over the past decade, according to Department of Health and Human Services figures.

Although many states increased their benefits during the 1970s, they failed to keep up with inflation. As a result, benefit checks bought less in 1981 than in 1969.

According to the calculations, in 1969 the average monthly benefit for a family of four without other income under the program of Aid to Families with Dependent Children was $558 measured in constant 1981 dollars.

By 1981, using the same constant dollars, the average per month had actually dropped to $394, a decline of 29.4 percent.

A separate HHS survey completed in May shows that at least one-third of the states trimmed benefits or restricted eligibility in 1981 for Medicaid, the federal-state program of medical aid to the poor.

According to the survey, 16 states adopted co-payments which require low-income patients to pay part of drug, eyeglass or dental costs; 20 states eliminated or cut back some of their optional services under Medicaid; 14 reduced eligibility; 10 limited the number of hospital days for which they would pay; and 10 limited nursing home reimbursements.

The Medicaid changes reflect in part some of the program reductions authorized by Congress last year at the request of President Reagan. He has asked for a new round of both Medicaid and welfare changes this year.

Welfare benefits over the decade varied among states. In Tennessee, for example, the maximum benefit for a family of four in 1970 was $129 a month, according to the Congressional Research Service. This rose to $148 by late 1981, not adjusted for inflation.

But over the same period, prices as measured by the Consumer Price Index rose about 140 percent, so the Tennessee benefit fell further behind in purchasing power. The 1970 Arkansas benefit of $100 a month rose about 60 percent by early 1982 but still didn't keep up with inflation.

Even if the value of food stamps is taken into account, these families on average nationally are worse off, another set of HHS calculations shows.

In 1974, the combined monthly welfare and food stamp benefit for a family of four without other income was $632 in constant 1981 dollars. But in 1981, it was only $536, a drop of about 15 percent.