The Prudential Insurance Co. has given Montgomery County 371 acres of land to become part of the 500-acre lake planned for Little Seneca Regional Park near Boyds.

The donation, the largest in the 55-year history of the Maryland National Capital Park and Planning Commission, will help create the largest county-owned lake and will be the focal point for Prudential's nearby 1,038-acre Water's Landing development.

Construction of the lake, expected to cost $31 million and take about two years after it begins this summer, was announced in March as a regional reservoir project.

Besides providing recreation and flood control benefits, the lake's 4 billion gallons of water could be released into the Potomac River during a drought, providing a 20- to 30-day supply of water to the Washington area. The lake is being built by the Washington Suburban Sanitary Commission, with the District of Columbia paying 40 percent of the costs and Fairfax County 10 percent. All draw drinking water from the Potomac.

The 371 acres dedicated to the county by Prudential represents about 20 percent of the 1,886-acre regional park. About 95 percent of the parkland has now been acquired, except for 112 acres, of which 70 are expected to be dedicated by other developers. The land being given is earmarked as open space and cannot be developed.

Prudential, one of the county's largest developers, will have about 3,600 housing units at Water's Landing. The project, about half complete, is being subcontracted to various other developers.

The lake, in which fishing and boating will be permitted, will be larger than lakes Needwood and Frank, the county's two flood-control lakes, but smaller than the two 800-acre Patuxent River reservoirs--Duckett and Triadelphia--owned by the WSSC. There will be two entrances to Little Seneca Regional Park when it opens, from Old Baltimore and Clarksburg roads.

A 90-acre lake was opened to the public recently at nearby Seneca Creek State Park, the huge 6,600-acre park being created by the Maryland Park Service. The park will stretch 12 miles from Interstate Rte. 270 to the Potomac when it all land purchases are completed.