Arthur Hull Hayes Jr., commissioner of the Food and Drug Administration, told a congressional subcommittee yesterday that his agency is taking steps to cut the review time for important new drugs by 25 percent over the next three years.

Hayes' appearance at a hearing of a House Science and Technology subcommittee came after emotional testimony by a 21-year-old woman suffering a rare form of encephalitis. Physicians believe she has been kept alive for a decade by the drug isoprinosine, which has not yet been approved by the FDA.

The FDA is charged with ensuring that drugs sold in the United States are safe and effective. The agency agrees that isoprinosine is safe, but FDA statisticians challenge the analysis used by the drug's manufacturer to prove its effectiveness.

Another witness, Edward A. Densmore, deputy director of the General Accounting Office, said that since the mid-1970s the FDA has cut six months off the average time it takes to approve all new drugs, but has increased the time it takes to approve new drugs for heart, kidney and cancer patients.

The FDA commissioner's plans for speeding drug approval include:

* Suggesting revisions in the legislation authorizing the FDA's approval process, including possible "breakthrough provisions" offering shortcuts for new drugs "that represent a major therapeutic advance for a life-threatening or severely debilitating ailment."

* Rewriting FDA rules on new drug evaluation. (The revisions are due out next year.)

* Closely monitoring FDA evaluation procedures to ensure that important drugs are given priority.

* Appointing a task force to consider a range of issues, including whether foreign studies alone could be used as a basis for drug approval and whether private institutions' review committees could take over FDA's role in monitoring early stages of the drug-testing process.