Private capital investment in Montgomery County, most of it in Bethesda and in the upper county along the Rte. I-270 corridor, has reached a record high of $952 million, County Executive Charles W. Gilchrist and Economic Development Director Ioanna T. Morfessis announced this week.

At a news conference Tuesday, the two officials said $161 million in new construction projects helped lower unemployment in the county to 2.5 percent in the last quarter of the fiscal year ending in June.

"Everyone has benefited from an unprecdented economic growth," Gilchrist said.

Gilchrist added that local commerce and industry contributed a "substantial" 27.5 percent to the county's total tax base. Businesses paid $3.2 billion in taxes this year, compared with $1.9 billion in 1979, when Montgomery launched its first comprehensive economic development program.

Morfessis announced that the county has allocated between $15 million and $20 million to develop the Shady Grove Life Sciences Center, Montgomery's centerpiece of public-private development. It also will be the site of the Center for Advanced Research in Biotechnology, a joint project of the University of Maryland, the National Bureau of Standards and the county government.

Morfessis added that she hopes to attract a major university, "not a full college campus but a graduate degree-granting institution," to the life sciences development area. County Looks For New Lobbyist

Montgomery County Council members are looking for a new legislative lobbyist to work at the General Assembly in Annapolis.

Patricia Billings, the first appointed to that job one year ago, will leave the post Oct. 31 to become chief lobbyist of the Washington-based Group Health Association of America.

The private group is the leading trade association for health maintenance associations across the country and is directed by James Doherty, a former chairman of Montgomery's Democratic Central Committee and a longtime Billings friend.

Billings said she was leaving the council staff position "because I really couldn't turn this new job down." Council members are expected to select Billings' successor between Oct. 15 and the end of the month. Hispanics Plan GOP Election Ads

A new brand of paid political announcements will soon hit the airwaves in Montgomery County. The local Hispanic Republican Club has produced the first political advertisements ever to air in the Washington area on Spanish radio, a club spokesman said.

Armando M. Lago, a Bethesda businessman, said the four spots will start Oct. 22 and will urge Hispanic voters to "vote Republican."

The four 30-second spots will focus on jobs, inflation, democracy in Central America and the election of County Council members by district. The latter appears as a referendum question on the November ballot and is favored by some Hispanics and other minorities.