The snow was coming down at a rapid rate, visibility was near zero and the cars on Lee Highway were barely moving. It was not a night to make a fast trip to the drugstore -- or a fast trip anywhere, for that matter.

But a steady flow of cars turned off the highway and into a snow-covered parking lot, where a huge American flag, caught in the spotlights, whipped wildly in front of an imposing concrete building.

It was hours past closing time for most stores and businesses but the service windows inside the Merrifield Post Office were open. In fact, there was a waiting line at the counter.

What brings people to the Merrifield Post Office on such an awful night?

Simple: It's open, unlike any other post office in Virginia. And it stays open for business until midnight, Monday through Friday, a special schedule post officials set last year for customers too busy to use smaller facilities during regular working hours. In addition, the Merrifield branch is open on Saturday, from 8 a.m. to 8 p.m., and on Sunday, 9 a.m. to 5 p.m.

Gerald F. Merna, manager of the Merrifield facility and an 18-year veteran of the postal service, said he initiated Merrifield's expanded service hours because he empathized with "all those people who work in all those restaurants, nightclubs and businesses that operate on two and three shifts. It's time we started to think about people who work long hours. I just got tired of people not being able to come until after work to get postal service."

The three-story post office is the largest central mail station in Virginia and, with the help of about 2,000 employes, it handles between 4 and 5 million pieces of mail in a 24-hour day.

Merna explained that his operation handles the mail for a 4,000 square-mile area: from the Potomac River south to Quantico, west to Winchester and over to the West Virginia border.

"All the mail from the whole world coming to the Northern Virginia area comes through my facility," said Merna, a former marine. "And all the mail from the area's 13 independent counties and cities comes through to go out to the rest of the world."

The Merrifield facility, located at the busy corner of Lee Highway and Prosperity Avenue in Merrifield, keeps two service windows open during its weeknight hours to serve the steady trickle of late mailers who come by.

Carol Greer, superintendent for window services, said business on Sunday is booming.

"It's ridiculous as far as customers go. I'm going to have to add additional clerks . . . it's just getting to that point," Greer said. She noted that the Sunday rush "usually starts when church is over."

Huong Luong, a window clerk working a recent snowy late night shift, said many people rush in minutes before midnight with bill payments that need to be postmarked before the next day.

One woman in a floor-length fur coat frantically stuck stamps on a handful of credit card payment envelopes.

"I haven't had time to get my bills paid and I had to get them in the mail by today," said the woman, who did not want to be identified. "This place is key to my life style because my busy schedule doesn't always allow me the luxury of stopping at the post office during the day."

Another customer, Jimmy Richmond, stationed himself in the post office's lobby and carefully pasted stamps on a large pile of white business envelopes.

"This place is the greatest thing in the world. I'm on the road from 6 a.m. until 7 p.m., and I don't have time during the day to mail things," Richmond, a footwear salesman, said between stamp licks. "Mailing things here at night makes me more professional because I can get orders to local customers by the next day."

Officials estimate that 40 percent of the local mail collected from Northern Virginia mailboxes or mailed directly from the Merrifield facility is delivered to recipients by the next day. The mail passes through Merrifield's mammoth 10-acre processing floor, which has room enough to park a couple of jumbo jets and then some. Robot-like tractors roll down wide aisles hauling bundles and boxes between sorting areas; mail clerks, wearing headphones plugged into miniature radios stuffed in their pockets, expertly sling letters into rectangular mail slots in split-second intervals.

"We have people working here with degrees in psychology or music," said Frank Tread, Merrifield's distribution manager. "It's a lot better paying than jobs on the outside."

Last month Merna kept customer service windows open for business on the President's Day national holiday, the first time he had done so.

"We get so many people who have to work when everybody else is off, I opened up," he said.

Merna has also opened a glitzy stamp collecting center, to the left of the main lobby, where philatelics can purchase the latest limited edition or special issue postage stamp from noon to 8 p.m., Monday through Friday. And the main lobby area is opened 24-hours so customers with special locked mail boxes can pick up packages or letters at their convenience.

The Merrifield Post Office also offers a special guided tour program. For information, call 698-6320.