The corner of 11th Street and Pennsylvania Avenue was once a centerpiece of commercial and social activity.

On the east corner was Kernan's Variety and Burlesk Show Theater; on the west, auctioneer Thomas Dowling had his storefront, and next door was the old Globe Theater. Behind them, along the present-day Mall, sprawled scores upon scores of small manufacturing plants.

The year was 1888, and Sen. Leland Stanford, chairman of the House Public Buildings and Grounds Committee, was on a carriage ride down the avenue searching for a new site for the U.S. Post Office's national headquarters.

He settled on that southwest corner, officially Square 323.

The buildings on the west side were razed, and workmen were digging the foundation for what in 1899 would turn out to be one of the city's most stately buildings. Topped with a 315-foot clock tower, the building would house the post office until 1934 when it needed more room and moved to new quarters adjacent to Union Station.

Meanwhile, around the old Post Office building, the Federal Triangle of buildings was growing: The Federal Trade Commission, the Commerce Department, the Internal Revenue Service, the Justice Department all were built on the property from Constitution to Pennsylvania and from 6th Street to 14th. From the 1930s until 1975, the old Post Office was a repository for small federal office units.

Now, through the ambitious design by the General Services Administation and Evans Development Co. of Baltimore, the two intensive uses--as a commercial commerce center and as a federal office building--are being melded into one. On Tuesday, Vice President George Bush will be on hand to rededicate the building at a gala ceremony that will include the first ringing of America's Bicentennial gift from England--10 bells donated by the Ditchley Foundation.

This is the first major federal project under the 1976 cooperative use law that provided for joint development of government-owned buildings with the aim of making them centerpieces for the community.

Under Evans' plan, the old Post Office's retail space on the ground, first and balcony floors around the 10-story atrium will include 5 restaurants, 9 high-priced speciality shops and a dozen low-priced "impulse" shops. There will be 16 fast-food operations in the Cookery emphasizing American cusine along a row called Main Street, U.S.A., and an international flavor in a section called Embassy Row. There will be no McDonald's or Burger King, only local entrepreneurs, Evans said.

Evans said leases will require store and restaurant owners to keep some of the facilities open from 8 a.m. to 9:30 p.m. as a drawing card for that part of Washington. Although it's on a strip of commercial properties that includes new office space, a Marriott Hotel, and the renovation of the Willard Hotel and the National Theater, some local eateries and small retail establishments have had trouble making it--there's little after-work activity in that part of the District.

"That's why we won the contract," said Charles C. G. Evans Jr., president of the firm responsible for leasing the space. "We had the most creative concept for helping to make this a building that blends with its surroundings; that draws people into it."

Based on what he calls conservative estimates, Evans expects one-tenth of 1 percent of all Washington tourists to come to his retail space, which he calls The Pavillion at the Old Post Office. He expects visits from 5 percent of D.C. residents and one-half of 1 percent of all D.C. office workers. Those numbers mean that each store will do $363 worth of business for each square foot of space in the first year and $460 for each square foot of space by the third year, Evans said.

Visitors coming off the Mall will go up a grand terrace on 12th Street into the back of the building--where the old loading docks used to be. There, a two-story cafe will offer intimate dining as well as breakfasts and late-night drinks. Immediately beyond on this main level are 10,000 square feet of fast-food stands selected to offer a different, but complete, line of foods.

"We didn't want just a chicken stand, we wanted a businessman who would offer southern-fried, barbecue and many different varieties," Evans said. "And they all had to have an evening as well as lunch-time menu." Average lunch cost: $3 to $5 per entry.

Beyond, on the main level, is the atrium. Above that section is a vast expanse of open space broken only by the old girders that supported the postal supervisors' catwalks. You are now in an area cut out from what the old building once was on a floor that used to house boilers and equipment. Toward Pennsylvania Avenue, the small impulse shops line the sides of the building as the center area spreads into a stage surrounded by dozens of tables to eat the lunches or dinners you've just bought in the Cookery.

The 10,000 square feet of impulse shops will feature gourmet foods, candy, stuffed animals, gifts, wooden toys, hot-air balloons, boutiques, books and newspapers. Each shop will have 500 to 600 square feet. Evans said that stores in new shopping centers average 2,800 square feet; in older shopping centers, they generally occupied about 5,000 square feet.

"The idea is to have high-turnover products, not stockrooms filled to the brim," he said.

On the next level, a horseshoe that surrounds the stage, the nine higher-priced specialty shops will offer quality accessories such as perfumes, lingerie, leather goods, luggage, sporting goods and jewelry.

Also on this level will be two major restaurants at the Pennsylvania and 11th and 12th street corners of the building--both spilling out onto the sidewalk.

On the third and highest interior level, a smaller horseshoe will have the largest restaurant--with American fare ranging from $8 to $14 in price.

"We had to set precedents to complete this project," said James G. Whitlock, GSA's commissioner of public buildings in the region. "There has never been a cooperative venture such as this before."

In the building around The Pavillion, five federal agencies have been assigned office space that GSA has renovated. Tenants will include the National Endowment for the Arts (325 people in 63,000 square feet of space on five floors), the National Endowment for the Humanities (270 people on 53,000 square feet of space on five floors), the Institute for Museum Sciences (14 people in 3,600 square feet of space), the President's Council on Arts and the Humanities (11 people in 2,400 square feet of space) and the Advisory Council on Historic Preservation (30 people in 6,000 square feet of space). About 5,000 square feet of office space for a federal tenant remains.

A controversial aspect of the office space in the historic building is that GSA arranged through Congress to use modular systems furniture.

The mix is startling, but GSA's Rodgers Stewart says the energy savings is 50 percent. Although agency officials couldn't come up with numbers, they claim they were able to squeeze about 100 more people into the building comfortably because the systems furnishings take up less space than other kinds of office furniture.

GSA Administrator Gerald P. Carmen considered use of such furniture aesthetically unpleasing, but found that he could do nothing because of legislation passed shortly before he took office. Now, dropped ceilings in the central areas of office space (called a "cloud") and metal-topped desks are locked together in most office areas.

So far, there have been "no complaints, but we only have one tenant the Humanities group moved in," Whitlock said.

The government is especially happy that Evans is the contractor. The only serious competitor was Rouse Co., a developer of shopping-complex-type facilities.

"One bid came in explaining how they could fit their standard concept into our building," Whitlock said. "Evans came in telling us how they can fit our building, where it is, how it is built and how it looks, with a concept that will draw people into it."

"We don't feel we were competing against anybody except ourselves," Evans said. "We feel it is our specialty to develop small urban centers." The old Post Office has just 56,500 square feet of leasable space.

Evans started leasing space selectively in January, 1982. Now, 32 of the 46 sites are leased, but Evans isn't saying specifically to whom--hoping for a big press bash later this spring. The opening of the retail space is now set for Sept. 7--rolled back from early this year, then mid-1983, because of construction delays in the building. It will be fully occupied by its federal agencies long before then.

The building itself won't be renamed, and The Pavillion at the Old Post Office title that Evans has adopted attests to that. So, to secure a reason for the handle, GSA has invited the U.S. Postal Service to set up a philatelic collector's stamp unit in the entrance hall to the building.