Transportation Secretary Brock Adams approved yesterday a $327.9 million federal grant to continue construction of two Washington Metro subway lines.

In a brief signing ceremony attended by local officials, Adams said he was "pleased" to release the money, particularly, he said, because from "time to time it seems that we're critical of mass transit."

The money was originally designated for the construction of interstate highways in the District of Columbia and Maryland. Under a federal law passed in 1973, such interstate money can be transferred to mass transit systems if the local officials so request and if the Dpartment of Transportation approves.

The money will provide for extending the National Airport line 4.5 miles south through Alexandria to Huntington, just south of the Beltway in Fairfax County, and continuing construction on the red line from Tenley Circle NW to Shady Grove, at 1-270 in Montgomery County.

Accompanying the grant were two federal strings requiring engineering studies of the feasibility of using different metal for the third (electrified) rail and the feasibility of different station designs in Montgomery County.

The federal money will pay 80 per cent of the construction costs; 20 per cent has already been provided by area jurisdictions.

In another Metro funding matter, House-Senate conferees agreed this week to include a $12-2 million payment of Metro's interest of construction bonds in a appropriations bill. The Senate had included the payment, technically due on Friday, and the House conferees accepted it.