In California the Little League has to pay to use public softball fields, and ill or injured persons taken to the hospital by city ambulance must pay for the ride.

In Detroit, Mayor Coleman Young wants to charge 50 cents to visit his city's aquarium, which has been free during the more than 30 years of its existence. In Washington, Mayor Marion Barry's proposal to impose a fee on those who swim in city pools or play tennis at city courts has raised a minor furor.

With their financial conditions worsening and voter sentiment against tax increases hardening, many cities are creating new sources of revenue by charging a fee or levying a tax for city services that were previously considered "free," that is, they had been paid for by taxpayers out of a city's general fund.

"It is really incorporating a private market principle into the government sector," said John Peterson of the Government Finance Research Center here. "If you don't pay, if you can't pay, you don't get the service."

A study Peterson helped prepare for the congressional Joint Economic Committee estimated that these charges have increased by up to 20 percent in cities around the nation, particularly in the West and Northeast. The report predicted that these user fees would continue to rise as local governments contend with the ever-more-difficult problem of balancing books.

In their survey of 594 cities, large, medium-sized and small, the Joint Economic Committee found that more than half expected to have budget shortfalls this year, with the likely result of higher taxes, cutbacks in services or layoffs of workers.

The survey was conducted before the Reagan administration's cuts in federal aid were proposed, and committee staff predicted that the situation will worsen, particularly for the largest cities that rely most heavily on federal aid.

The committee report was critical of the federal government for failing to develop a policy to help cities on the brink of default.

"You have something approaching a crisis, not just for one or two, but for scores of cities," said Rep. Henry S. Reuss (D-Wis.), committee chairman.

The survey noted that contracts for more than half of the employes in big city police, fire and sanitation unions expire this year, and said there would be added pressure on city budgeters as these workers press for cost-of-living salary increases to counter the effects of inflation.

Reuss predicted that the effect of these financial pressures would be more striking city workers, more uncollected garbage and trash, more unrepaired potholes and streets, more utility brownouts and more crumbling bridges.

"In short," he said, "the beginning of America in ruins."

The suffering of cities like Boston, New York, Washington, Detroit and Chicago is well known, but the committee's survey found that the problems were spread around the country and not confined to the industrial north.

Urban experts say older cities in the Sun Belt, such as Atlanta, Birmingham and New Orleans, also are experiencing financial problems.

California, now feeling the effects of tax-restraining Propostion 13, is perhaps the most creative in coming up with new sources of money.

The multibillion-dollar state surplus that had triggered the taxpayer revolt has been spent, for the most part, and local governments are pioneering the concept of user fees to raise additional revenues.

In the fast-growing city of Vacaville (pop. 43,000), outside Sacramento, sports leagues are charged $1,000 a year to use playing fields. An ambulance ride to the hospital costs $50. The buyer of a new home faces an array of charges, adding an average of $6,000 to the cost of the house, to help cover costs of fire stations, schools, parks, water and sewer.

The new fees are being challenged in courts by some who contend that they are a back-door way to get around Proposition 13's requirement that no new taxes be imposed without a two-thirds majority vote in a municipal referendum.

Aides to Washington's Barry have been reviewing for more than a year the 1,100 user charges on the books. Parking fines have been raised, as have fees for liquor licenses, building permits and professional licenses for doctors, barbers and accountants.

Copies of birth certificates, priced at a dollar for more than a decade, now cost $3. And the proposal for user fees at swimming pools and tennis courts is still being considered.