The Health and Human Services Department said yesterday that the nation's 30 million Medicare beneficiaries will be eligible next month to join prepaid health plans, some offering free extras such as eyeglasses or prescription drugs.

Under rules to be published in the Federal Register today, the health plans will collect a monthly fee from Medicare -- and in some cases a small premium from beneficiaries -- in return for providing comprehensive hospital and doctor coverage at least equal to Medicare's.

The government estimates the rules will encourage 200,000 more elderly people to join health maintenance organizations (HMOs) or other prepaid plans by next year. Currently, 400,000 are enrolled.

"Under these rules, HMOs can offer Medicare beneficiaries substantial benefits not covered by Medicare, in some cases with the added incentive of reduced out-of-pocket costs," HHS Secretary Margaret M. Heckler said in a statement.

Spokesmen for HMOs said most prepaid plans would be interested in signing up Medicare beneficiaries, some immediately, others within a year or two. The government expects the program to be in high gear by midyear.

"We feel it will be a very successful program," said Peggy Pond of the Group Health Association of America. She called the new rules "perhaps the most significant step for health care for older Americans since the enactment of Medicare itself." HMO members will receive all medical care guaranteed by Medicare without paying the usual hospital deductible or 20 percent share of medical bills.

But they must use the HMOs designated doctors and hospitals, except in emergencies.

Some HMOs may charge small monthly premiums. The Group Health Association of America estimates that premiums will average $15 a month. HMO members also would continue to pay the $15.50 monthly premium to the government for doctor bills.

HMO enrollment is voluntary and recipients can return to the Medicare program on short notice at any time.