In the 20 days the Carter administration has left in office, the Justice Department is considering two major lawsuits to force the busing of students in metropolitan St. Louis and metropolitan Kansas City, Mo.

Sources said no decision has been reached to file either suit but the St. Louis case is understood to have reached the desk of Associate Attorney General John H. Shenefield, who must pass it on to Attorney General Benjamin R. Civiletti for final approval. The suit is understood to seek to integrate the predominantly black city schools with predominantly white suburban ones.

Sources say the only reason the Kansas City suit has not gone to Shenefield is that the Civil Rights Division does not want to burden him with two major school suits at the same time. One source said there is also some concern over the political fallout from two busing suits against school systems in the same state at the same time.

The St. Louis and Kansas City suits, if they are filed, would join a steady stream of civil rights cases coming out of Justice as the Carter administration nears its end. Just yesterday, Justice sued the Indiana prison system and the small town of Secaucus, N.J., (58 policemen on its force, no women) for discriminating against women in their hiring practices.

And in the last month, Justice has also sued:

The Los Angeles Police Department and the South Carolina state police seeking to end what it called their past discriminatory hiring practices.

Troup, Tex., charging that the city violated the Revenue Sharing Act by not providing its black residents the same street paving, water and sewer services it gave its whites.

The public schools of metropolitan Charleston, S.C., charging that the 20 school districts in and around Charleston are segregated. Justice said there are fewer than 50 white students in a city school population of more than 8,000.

The plan to sue the public schools of St. Louis County is understood to charge that blacks make up only 22 percent of the suburban school population of 150,000 and 77 percent of the city school total of 62,000. One Justice source said that black students have been effectively kept out of suburban St. Louis schools for the last 15 years.

"The same thing is true in Kansas City," this source said. "And if you can't send your children to schools in the suburbs, you won't push hard to move to the suburbs either. What we've got here is a very strong school and housing lawsuit."

The department is also understood to have ready a plan to file a housing discrimination suit against Memphis. Sources said a lengthy investigation has revealed that Memphis has not used federal funds to finance any public housing in the last 10 years even though funds have been available.

"There is a real departmental fight going on over whether this suit should be filed," one source said. "There is also a war going on between Justice HUD [the Department of Housing and Urban Development] over who would handle such a suit."

Justice is believed to have dropped plans to file discriminatory housing suits against three surburban communities -- Rancho Palos Verdes, Calif., the New Haven suburb of Orange, Conn., and the Rochester suburb of Pittsford, N.Y.. One source said all three cases would have been hard to prove since they involved zoning practices that have been standing for years.

Civil rights groups fear that the incoming Reagan administration may not be as aggressive as the Carterites on desegregation and discrimination questions.

In looking back over his four years as head of the Civil Rights Division, Drew S. Days III recently said he was proudest of the fact that the public identified the Carter administration with justice in the civil rights arena.

"I get the impression that we're regarded as standing for something," Days said in an interview. "We're not neutral, we're advocates, we're there when there's a need. We're out there ferreting out violations and prosecuting people."