The Gilbane Building Co. has begun construction on the first of eight buildings at Willow Oaks Corporate Center, a 35-acre business park developed by the Prudential Insurance Co. of America at Rte. 50 and Gallows Road in Merrifield.

The first building, scheduled for occupancy next summer, will have eight stories totaling 191,000 square feet, with from 19,000 to 24,500 square feet per floor.

* The American Association of Retired Persons has leased 68,000 square feet of space at 1133 20th St. NW, in addition to its headquarters at 1909 K St. NW.

The 20th Street building, which was designed by Skidmore, Owings and Merrill and constructed by Omni Construction Inc., is a joint venture of Northwestern Mutual Life Insurance Co. and the John Akridge Co. The building is scheduled to open in February.

* Watson & Taylor Realty Corp. recently purchased a 5.8-acre site in the Hampton Business Park in Capitol Heights for $900,000, the Kenneth H. Michael Cos. reported. The Dallas-based developer plans to build an 80,000-square-foot office and warehouse complex on the site.

* Brewster Wallcovering Co. in Boston has leased 12,978 square feet of warehouse and showroom space on West Street in the Ardwick-Ardmore section of Prince George's County, a commercial warehouse district near Cheverly.

* Autometric Manufacturing Inc., an electronics manufacturing subsidiary of Falls Church-based Autometric Inc., has become the first resident of the 34-acre Stanley Industrial Park in Stanley, Va. The company has constructed a new 21,000-square-foot facility there, with 6,000 square feet of office space and 15,000 square feet dedicated to manufacturing.

* Long & Foster Realtors has sold office space at Merrion Square, 7353 McWhorter Place in Annandale, a 36,000-square-foot office building scheduled to open in the spring.

The transactions included 2,400 square feet to the Anderson Burnett Corp. and 2,000 square feet to Charles Gibson.

Monarch Life Insurance Co. has leased 3,000 square feet in Commerce Center at Reistertown Road and the Beltway.

* Computer Sciences Corp. has leased 35,260 square feet at 8229 Boone Blvd. in Vienna.

* Nussbaum, Owen & Webster has leased 25,096 square feet at One Thomas Circle NW.

* Capital Systems Inc. has leased 25,000 square feet at 1529 King St. in Alexandria.

* Claritas Corp. has leased 21,219 square feet at 201 North Union St. in Alexandria.

* First American Bank Inc. has leased 18,051 square feet at 734 15th St. NW.