After months of speculation, two new names have emerged as the leading candidates for commissioner of the Social Security Administration, to replace Acting Commissioner Martha A. McSteen.

They are Sylvester Huse, 61, of Morristown, N.J., a former high-level data-processing official and planner at American Telephone & Telegraph Co., and John Reynolds, 52, a vice president of International Telephone and Telegraph Corp., from Allendale, N.J.

Although Department of Health and Human Services Secretary Margaret M. Heckler originally sought the nomination of McSteen, a career official, the White House never cleared the nomination, reportedly because it wanted to choose a candidate of its own with strong data-processing credentials.

At one point the nomination was considered all but wrapped up for Dorcas R. Hardy, assistant HHS secretary for human development services, but support for her faded. Another potential nominee, Sandra Smoley, a local official in Sacramento, Calif., and former president of the National Association of Counties, reportedly has withdrawn from the running, possibly because she wants to run for higher office.

Meanwhile, Pete V. Hererra, a computer expert from Texas, has been named associate commissioner for information systems. Bob Robinson, long-time editor of the Social Security Bulletin, the "bible" for statistics and analyses of agency programs, has retired.

MEDICAL COST INFLATION . . . The nation may be making some headway in the battle against rising health-care costs, but Democratic experts warned at a recent briefing that it is too soon to celebrate victory.

The medical care component of the Consumer Price Index (CPI) rose 6.2 percent in 1984 -- the fourth successive decline in the rate of increase since the high of 10.9 percent in 1980. And HHS estimates that total national health expenditures -- reflecting increases in the volume of services, population and prices per unit -- rose to $384.3 billion in 1984, an increase of 8.1 percent over 1983. In 1980, that figure was 15.3 percent; and last year, it was 10.3 percent.

Furthermore, the American Hospital Association reported that 1.5 million fewer patients were admitted to hospitals in 1984 than in 1983 and that the average stay dropped from about 7 days per patient in 1983 to 6.7 days in 1984.

As these trends emerged last year, Heckler declared that administration policies, particularly the prospective payment system for Medicare, were breaking the back of long-term medical inflation.

Karen Davis of Johns Hopkins, Theodore R. Marmor of Yale and several other health experts on a panel convened last week by the Democratic-learning Center for National Policy, said the prospective-payment system appears to be helping control Medicare costs. But several argued that the statistics might simply reflect the drop in inflation for the entire economy. About three-fifths of health-care inflation is usually thought to be a product of overall inflation.

Several members also noted that the medical component of the CPI, at 6.2 percent, rose 1.5 times the rate of the overall CPI, close to its historic ratio. They said this suggests that if overall inflation rises again, medical inflation also could spurt upward. Davis said one major reason the rise in hospital costs has slowed is a sharp reduction in hospital admissions for non-Medicare patients that could not have been caused by the prospective-payment system.

HEALTH CARE COVERAGE . . . The Census Bureau, in a recent report on health-insurance coverage for the poor, said a survey in the fourth quarter of 1983 showed that 84.8 percent of the population had some form of government or private health insurance, leaving 35 million people with none.