After Barbara J. Toney's landlord failed to kill the mice and roaches in her two-bedroom apartment, fix the falling plaster and replace the missing doors, she decided to fight back.

She stopped paying her rent.

And until her battle with the District of Columbia government was settled a few months ago, Toney was one of the 5,000 city public housing tenants who owed back rent -- in her case $6,000 over the course of six years.

Nearly half of the residents of the city's public housing projects owe the city rent -- to the tune of $1.2 million, or 10 percent of what rent collections should be. More than 70 percent of the amount has been due for more than three months.

The problem is so widespread that the federal government, the source of most of the money for running the projects, has set a Sept. 30 deadline for the city to improve its rent collections substantially.

The squable between Toney and the District government is replayed hundreds of times throughout the city's 52 public housing projects until a mean cycle has been created.

The city defers making the small repairs tenants request because it doesn't have the money. Many tenants refuse to pay their rent because they can't get repairs made. And when the city can't get the rent money, it falls further behind in making the repairs.

And there is an added twist -- the city rarely evicts those who don't pay. Most public housing tenants -- and most of those who are delinquent -- are single mothers like Toney, with several children, who usually live on less than $5,000 a year and couldn't afford other housing if the city put them out on the street. Only 25 families were actually evicted last year. These families had refused to pay even when a U.S. marshal turned up on their doorstep with a court order to force them out.

Some tenants, of course, don't pay their rent, not because of outstanding repairs, but to use the money for other things -- clothes, gifts for Christmas, liquor or drugs, they admit.

But, says Sidney Glee, the acting administrator of the city's public housing projects, "We don't really want to evict because basically we're the end of the line."

Because of so few evictions, however, "people have gotten the feeling that nothing will happen to us if we don't pay our rent," said Leola Bynum, president of the tenants association in the Kelly Miller project in Northwest.

The city's rental income, which totalled $9.7 million in the last fiscal year, is crucial for fixing minor maintenance problems in the District's decaying public housing stock.

The only other current source of operating funds -- a subsidy from the U.S. Department of Housing and Urban Develoment -- has been stuck at about $15 million for the past four years, increasing from $14.1 million in fiscal 1977 to $15.7 million this year.

"Since we get a fixed amount from HUD, when we have a shorfall from rents, we cannot do what we want to do, like buy new refrigerators and new stoves," said D.C. housing director Robert L. Moore.

"When they [tenants] don't pay rent, that really hurt everybody," because rent money also must pay for higher utility bills and salary increases for public housing employes, he said.

Under law, public housing tenants must pay 25 percent of their income for rent, so rent payment can only rise when tenants' income rises, and they decrease if their income drops.

While the District fails to collect 10 percent of its public housing rents, New York City, which has the nation's largest public housing authority, and Chicago, the second largest, lose only 3 and 4 percent respectively.

Instead of evicting delinquent tenants, city housing officials try to recapture some of the unpaid rent by filing lawsuits against them.

After the housing department filed such a suit against Toney four years ago, she countersued with the help of Lynn Cunningham, a Neighborhood Legal Services attorney who represents public housing tenants free of charge. Toney said she hadn't paid her rent because the city hadn't made the repairs she had requested.

A city housing inspector visited Toney's apartment and found 14 housing code violations there. And last October, a D.C. Superior Court jury decided she didn't have to pay any of the rent she owed because of the longstanding violations.

The $6,000 award was the largest rent abatement ever granted because of the city's failure to perform repairs, according to Cunningham. The city plans to appeal the decision.

In 30 percent of the 1,000 cases filed in court last year, tenants with lawyers got the city to agree to make repairs in return for partial repayment of rents.

In 60 percent of the cases, tenants reached agreements with the city without the help of an attorney; in those cases, they agreed to pay the rent owed.

Housing department attorney James B. McDaniel said that the difference in the settlements was because in most cases no repairs were necessary.But tenant attorneys said that tenants without counsel don't know their rights and don't force the city to make the repairs.

Officials in the local HUD office, as part of a national policy to make public housing less of a drain on the federal treasury, have told city officials to reduce the rent backlog in the largest projects.

The city has until Sept. 30 to reduce the level of lost rent in 26 large projects down to the city wide level of 10 percent. For example, in the project with the worst record, Frederick Douglass in Southeast, residents owe more than two times the amount -- 219 percent -- of what the city should collect in a month.

HUD has not said, however, what will happen if the city can't make that deadline.

Under a new policy established with HUD's prodding, once a tenant fails to pay his rent, the city will go to court against him, rather than letting the tenant work out an agreement with his project manager. And now only housing department lawyers will be allowed to make such agreements.

In the past, the project managers often failed to enforce the agreements and tenants would fall further behind in their payments.

In addition, the city's housing department has also created a special task force of maintenance workers to try to make repairs so that tenants can't use lack of repairs as a defense if they fail to pay their rent.