Metro subway service will be extended from Farragut North to the Dupont Circle station starting next Monday with the first train leaving about 6 a. m. the transit authority has announced.

The extension will add about one half mile to the existing route, making it 5.1 miles long instead of the current 4.6 miles.

Farragut North, located beneath Connecticut Avenue and L Street NW has been the interim terminal of the first line since public service began last March.

At the outset, only the north entry of the Dupont Circle station - at Connecticut Avenue and Q Street NW - will be opened. Opening of the south entry, in the Dupont Circle building on the south side of the Circle, will be delayed several weeks.

Anthony J. Stefanac, general superintendent of rail operations for Metro, said the lengthening of the line will require the addition of one more train to maintain in both rush-hour and midday service. Operations will continue to end at 8 p. m.

During rush hours, seven trains will be operated instead of the current six, Sefanac said, while in offpeak periods, four trains will be operated instead of the current three.

With the Dupont Circle station opening, Metro will have seven stations in operation on its first line. The Gallery Place station at 7th and G Streets NW, which remained closed when service began last March, was opened last month.

Metro's second route, running from the Stadium-Armory station through downtown Washington to National Airport, is scheduled to begin operations next July 1. Test trains already are operating through the Potomac River tunnel to and beyond Rosslyn.