Here are highlights of contingency plans, compiled from federal agencies still sorting out what services would continue:

SOCIAL SECURITY The government would continue to make Social Security payments to the 53 million beneficiaries. “We will continue to process applications for benefits, but it might take longer if a shutdown does occur,” said Mark Hinkle, a spokesman for Social Security. “Our local offices will open for limited services. We are working on the specifics.”

A huge backlog of applications for Social Security disability benefits would grow even larger, agency officials said.

HEALTH CARE Medicare, the program for people who are 65 and older or disabled, would continue to pay doctors and hospitals for several weeks, using money from its trust funds.

The National Institutes of Health would not admit new patients to its research hospital or begin new clinical trials of drugs, devices or other treatments. However, research studies already begun would continue.

TAXES. The Internal Revenue Service would not audit tax returns and would not issue refunds to taxpayers who file returns on paper. But the agency would process returns filed electronically, a majority of all filings, and would issue refunds for those returns. Douglas H. Shulman, the I.R.S. commissioner, said federal taxes would still be due on April 18. (The deadline had already been extended three days because of a local holiday here.) The I.R.S. plans to keep enough people on hand to cash incoming checks because the government needs the money.