The District government and private lending institutions will invest $142.9 million over the next three years to build or rehabilitate 6,200 housing units, most of which will be sold to low- and moderate income families, Mayor Marion Barry announced yesterday.

The program will enable some of the city's less affluent residents who are hardest pressed by soaring area housing prices to buy 1,613 new or remodeled homes and apartments -- with no money down -- for less than their market value.

Another 3,820 units will be rented eventually to low- and moderate-income families with annual incomes of between $17,000 and $22,000, Barry said. Rents will run from $120 to $600 monthly.

The remaining 769 units will be sold on the open market and are clearly intended to attract affluent buyers into some of the city's poorer areas. Included in that category are three large city-owned Victorian homes in Shaw, just north of downtown, that will sell for more than $150,000 after they are restored, city officials said.

The announcement of the mayor's long-awaited plan comes at a time when low- and moderate-income families are being forced from the city's housing market by a combination of skyrocketing housing prices and a diminishing number of rental units.

Many houses, once the rental homes of the poor, are being sold for $100,000 and up to middle-class buyers eager to live in the city. The poor they displace are crowding into whatever living quarters remain where they are forced to pay ever-increasing rents or to wait as long as five years to move into public housing.

At the same time, apartment complexes that traditionally provided the poor and moderate-income families with reasonably priced housing over the years are being converted to condominiums that neither income group can afford. Inflation-swollen interest rates are further compounding the problem for those moderate-income families saving to buy a home.

Barry said the new program is "carrying out my mandate and my commitment to get the boards off (abandoned and dilapidated) housing and to provide increased housing opportunity to District residents, especially those who cannot afford housing in today's market."

Some 2,156 units are already under construction, Barry said, and of that number about 1,000 of them will be completed by the end of the year. Of those currently under construction, however, 1,065 are one-bedroom apartments that will be rented largely to the elderly. The city's most pressing need is for multi-bedroom units because so many of its low-income families are large, city officials said.

More than a third of the units to be built or renovated are in Southwest and in the neighborhoods north of downtown, officials said.

So much activity is concentrated in those neighborhoods, because, in the past, the city purchased large amounts of property there under its old urban renewal program.

Originally, city officials intended to tear down the old buildings they had purchased, but a lack of money and community opposition slowed the demolition.

In the intervening years the philosophy of urban renewal changed from demolition to rehabilitation and the city found itself the landlord of more than 1,000 vacant or partially occupied buildings that now will be restored.

Almost 20 percent of the new units will be in Anacostia, which contains one of the largest concentrations of poor in the city but little city-sponsored new housing. The new building plans call for restoring or building 1,532 new units there.

These plans include building a 400-unit mixed-income apartment and townhouse community at Knox Hill, site of one of the city's earliest public housing projects, which was torn down several years ago.

The city also plans to rehabilitate several large apartment complexes such as the 400-unit Congress Park, all of which have stood vacant and boarded up for years.

Under the housing program:

Two obsolete and vacant schools -- old Shaw Jr. High at 7th Street and Rhode Island Avenue NW and Carberry in Northeast -- will be converted into apartment units for lower-income families and the handicapped. Shaw, long known as "Shameful Shaw" when it was in operation because of its delapidated condition, would be gutted, but its turn-of-the-century exterior would be preserved.

Some $218,000 will be loaned to nine Seaton Street lower-income homeowners so they can renovate their homes;

Another $1.6 million will be loaned to tenants at the Kenesaw apartments in Mount Pleasant for the renovation of their complex which they bought from the former owner so they could convert it into a cooperative.

Other areas where rehabilitation and construction will occur are the 14th Street corridor and the Shaw section. The remaining units will be scattered throughout the poorer sections of the city, officials said.

In all, the city intends to build or rehabilitate 3,820 rental apartment units by 1982. The remaining 2,382 units will be set aside for homes ownership -- condominiums, cooperatives and single-family homes.

The more than 10,000 families now on the city's waiting list for public housing will be given priority in filling the rental units, said city housing director Robert L. Moore. As a result, he said, "we can substantially cut" the waiting list in the next year.

Moore said that, of the $142.9 million to be spent, $134.4 million will come from the private sector, including banks, savings and loans, and mortgage investment companies. The city will invest $18.6 million -- part of which will be repaid when the private money kicks in -- as leverage to attract the private investments, which will be largely guaranteed by the federal government.

The majority of the construction and rehabilitation work will be undertaken by private construction firms using the private money for financing.

Barry said many of the projects were put in the housing pipeline by his predecessor, Mayor Walter E. Washington, but "they never happened" or were not completed.

"This administration is putting into action what other people were just talking about," he said.

Scientists at both Argonne and Livermore filed affidavits supporting the Progressive's contention that Morland's article was no more secret than those written by government scientists for encyclopedias.

Argonne is operated for the Energy Department by the University of Chicago to do nuclear physics research. Livermore is run by the University of California at Berkeley and concentrates on nuclear weapons research. CAPTION: Picture 1, The old Shaw Junior High School at 7th St. and Rhode Island Ave. NW will be converted into apartments for physically handicapped and low-income tenants, Photos by Vanessa R. Barnes -- The Washington Post; Picture 2, Some homes, like this one on 13th Street, will be sold on the open market after being renovated, photo by Vanessa R. Barnes -- The Washington Post; Picture 3, House on 9th Street is expected to bring $249,000, Photo by Vanessa R. Barnes -- The Washington Post