Mary Zimmerman rushed out of her government office yesterday afternoon, dashed through the rain, and reached her destination just in time. She wasn't rushing to file her income tax return but to open an individual retirement account at her bank.

Zimmerman, like thousands of other area residents, met dual deadlines yesterday. It was income tax day and the last day to get a tax break for 1985 by opening or adding to an IRA.

For Zimmerman, a 36-year-old fiscal clerk, it was her first IRA. The retirement accounts have become increasingly popular because they represent a deduction of up to $2,000 and can sometimes mean the difference between a tax refund or having to pay.

"I had the stamp on the envelope and I was already to mail my return when one of my bosses told me about IRAs," she said. "So I ran over here to my bank. This means I will get an extra $600 back. Now I will mail my return."

Downtown at the Internal Revenue Service office at 12th and E streets NW, dozens of people crowded around 60 blue bins filled with various tax forms. The only bin that was empty was the one for automatic extensions that allows taxpayers to file after April 15.

"This year I am expecting to get some money back so I will file for an extension," said Michael Coates, 26, a Justice Department worker, as he searched for a replenished supply of the extension applications. "I think it's the weather. I just don't feel like doing it today."

One floor above, about 30 people waited for an IRS employe to help them with the forms.

Security guard Earl Enwkrirk, 24, said he waits until April 15 every year to come for help.

"You get the best help if you wait till the last minute," he said. "The IRS has all their information together on the last day."

Area banks and financial institutions surveyed yesterday reported sharply increased business in IRAs.

Roger Conner, a vice president of American Security Bank, said that the bank handled 500 IRAs on Monday and expected to exceed that number yesterday.

"Between April 3 and today, we did 50 percent of our IRA business for the year," he said. "And since noon, we have been busy in all of our 31 branches. What that tells me is that as a country, we are a group of procrastinators."

Chase Bank of Maryland also was swamped by last-minute IRA customers.

"We are extremely busy," said Vice President Kathleen Carpenito. "We set a goal of opening 5,000 IRAs and we did that a week ago. We expect to end up with 2,000 more accounts by the time we close at midnight tonight."

Carpenito said that 35 percent of the accounts were opened by telephone. "We try to make it as easy as possible for our customers," she said. "But even with a phone call, people will wait till the last minute."

The new IRA Store in Arlington, a private business that offers a number of investment options, was open through the weekend and was scheduled to stay open last night until midnight, according to President Lakshma Sampath.

"We opened 150 accounts on Saturday and Sunday and we expect to be busy right up till midnight tonight," said Sampath. "We find that 80 percent of our customers are investing in mutual funds. And that most have waited till the deadline to come see us."

Outside the Main Post Office at North Capitol Street and Massachusetts Avenue NE, 20 postal workers braved the thundershowers to allow passing motorists to dump completed tax forms into giant sacks. "It's a real crowd," said one Postal Service spokesman just before midnight. "But we can handle it."