Construction has begun on the $10 million Golden Triangle II office building at the planned six-building Golden Triangle Office Park in Greenbelt.

The seven-story building will contain 133,000 square feet of space. It is identical to the adjacent $7 million Golden Triangle I, completed in July 1982. The 37-acre planned office park on the Capital Beltway between Kenilworth Avenue and Greenbelt Road will contain 900,000 square feet of space when completed.

The park's developer is Prudential Insurance Co. of America. Builder for the first two buildings is Salus Corp. of Arlington. Occupancy of Golden Triangle II is scheduled for next spring.

Other recent developments in commercial realty in the metropolitan area:

* IPL Systems Inc. has leased 1,357 square feet of space for a sales office at 8201 Corporate Drive in Landover. The Waltham, Mass.-based firm manufactures, sells and services plug-compatible computers.

* Pabon, Sims, Smithy & Associates Inc., transportation safety and environmental engineering management consultants, has leased 1,411 square feet of space at 734 15th St. NW.

* E.G.&G. Washington Analytical Services Center Inc. has leased 5,555 square feet of space in the Airmen Memorial Building at 5211 Auth Road in Suitland.