Two candidates have emerged for the job of undersecretary of Health and Human Services now that Charles D. Baker has stepped down. John J. O'Shaughnessy, assistant secretary of HHS for management and budget, and Thomas R. Donnelly Jr., special assistant to the president for legislative affairs and a former chief lobbyist for HHS on Capitol Hill, are the people most frequently mentioned. Baker is vacating the post to join Northeastern University in Boston as a professor of business policy.

GETTING THE STATES TO PAY UP . . . For years, state governments have been paying the Medicare premiums for many low-income elderly persons, thereby making sure that most of their doctor bills would be picked up by Medicare, a federal program, rather than Medicaid, which is paid by both the states and federal government. Although this "state Medicare buy-in" is allowed under the law, states sometimes have delayed paying the premiums to the federal government. For example, according to a recent HHS memo, in mid-1984, Alabama was two years in arrears, owing the U.S. government $25 million.

In recent months, the Health Care Financing Administration, which administers Medicare, has undertaken a campaign to force states to pay promptly. As a result, as of April a total of $87 million in overdue state premiums had been cut to $4 million. In the future, states must pay premiums within 45 days after they become due or will be assessed interest on the balance and threatened with reduction of federal support for the state Medicaid program.

RETIREMENT ASSETS . . . Couples in which the husband is the prime breadwinner who retired on Social Security in the year beginning June 1980 had a median net worth of $68,300 in 1982, consisting of $48,300 in home equity and $20,000 in other assets, the Social Security Administration reported. Nearly nine-tenths were homeowners. The comparable net worth figure for unmarried men was $17,000, and for unmarried women, including widows, it was $30,000. In all cases, these assets were substantially more than the net assets of a group of retirees surveyed in 1969. That year the median net worth of couples in which the husband was the prime breadwinner was $42,300 in 1982 dollars.

SOUNDING OFF ON SOCIAL SECURITY . . . As the Social Security system celebrated its 50th anniversary last week, the American Association of Retired Persons released a poll showing that 92 percent of respondents consider the program a success and 90 percent oppose cuts in benefits as a way of reducing the federal budget deficit.

The Yankelovich, Skelly and White poll of persons 25 and over suggested that people who have not yet retired are most likely to be dissatisfied with the system; 40 percent of them call benefit levels "inadequate." A spokesman for the survey firm said these active workers are not worried about their future benefits so much as the benefits their parents get. "They don't want them moving in," she said.

Retirees seemed content with benefits: 55 percent said Social Security is their most important income source. Overall, 43 percent said they are comfortable financially and 41 percent said they have enough to get by; 16 percent reported serious financial difficulties.

MEANWHILE IN HEALTH CARE . . . The American Hospital Assocation reports that within the past year the four largest for-profit hospital chains -- Hospital Corp. of America, American Medical International, Humana and National Medical Enterprises -- have set up their own health insurance companies, enrolling about 1 million people. Voluntary Hospitals of America, a nonprofit group that includes about 450 hospitals, planned to join with Aetna Life and Casualty Co. to create a health maintenance organization and other service groups.