Internal Revenue Service officials said yesterday that tax return processing is still going smoothly, despite the influx of about 40 million tax returns as the tax season grinds to a close.

"The service centers are still upbeat and getting the stuff done. Everything is being processed as it comes in," IRS spokesman Rod Young said. However, the IRS cautioned taxpayers filing at the last minute who are due refunds that the wait is likely to be eight to 10 weeks rather than the six to eight weeks in which those who filed earlier have generally been receiving refunds.

To help taxpayers who have waited until the last minute to file their returns, the IRS and the U.S. Postal Service have set up special accommodations.

IRS offices will provide telephone assistance to taxpayers today until 6:45 p.m., and the IRS office at 1201 E St. NW in the District and the IRS office at 31 Hopkins Plaza in downtown Baltimore will be open until 6 p.m. for personal assistance to taxpayers who come by, IRS spokesman Dom LaPonzina said.

The telephone number for tax assistance in the Washington metropolitan area is 488-3100.

In addition, the IRS will have personnel on hand in the lobby of the Main Post Office at North Capitol Street and Massachusetts Avenue NE until 11:30 p.m. tonight to answer questions and offer assistance, LaPonzina said. Tax forms also will be available in the lobby, he said.

The lobby of the Main Post Office will be open until midnight, as it is every night. Late filers will be able to purchse stamps, money orders and to send their tax returns certified mail, if they choose, Post Office public affairs officer George Conrad said here.

In addition, uniformed letter carriers will be stationed for about a block in each direction from the post office to receive tax returns that are completed and that have correct postage.

In suburban Virginia, the Northern Virginia Sectional Center in Merrifield in Fairfax County also will stay open until midnight with letter carriers stationed outside to receive tax returns. The Old Town Post Office in Alexandria will close its window service at 5:30 p.m. as normal but will make a midnight collection from the boxes in front of the post office.

In suburban Maryland, several post offices will be open until midnight, including the Bethesda Post Office, the District Heights-Forestville Post Office, the main post offices in Riverdale, Silver Spring, Bowie, Laurel and Rockville and the Waldorf Post Office.

LaPonzina said that the IRS regional offices yesterday had handled about 12,000 telephone inquiries and had helped about 500 taxpayers who walked into the offices as of about 4 p.m. yesterday.

"There are still about 30,000 D.C. taxpayers and about 300,000 Marylanders who will be dropping their returns in the mail," he said.

IRS Commissioner Roscoe L. Egger Jr. told the Economic Club of Detroit yesterday that by the end of the century, taxpayers will figure their taxes on home computers.