In fortified apartments scattered about the flat expanse of south-central Los Angeles, "rock cocaine" is selling like crazy, heralding a marketing breakthrough that furnishes this middle-class drug to the city's poorest neighborhoods.

The hard little pellets of cocaine powder, selling for as little as $25, have not been found in significant numbers outside Los Angeles, narcotics experts said.

"Even if we put a thousand people using it in jail, twice as many would come out on the streets looking for it," said Enola Byrd, a staff psychologist at the Jamaa Drug Treatment Center here. With rock cocaine's growing popularity and what Byrd considers glamorized cocaine use on television and in movies, 90 percent of the 250 people his center admitted in the last six months were cocaine users, a 70 percent increase over a year ago.

Rock cocaine provides a cheap method of "free-basing," heating the drug to release a vapor that can be smoked through a pipe. Most users inhale powdered cocaine into their noses, but at $100 to $125 a gram this can be expensive.

The mushrooming sale of rock cocaine appears to be part of a national increase in cocaine use by every income group, after unusually large shipments of the illegal drug forced down the price, according to Nicholas Kozel, an epidemiologist for the National Institute on Drug Abuse in Rockville.

Bobby Sheppard, intelligence unit supervisor for the U.S. Drug Enforcement Administration here, said he saw no sign of rock cocaine spreading to the rest of the country, "but I've got to believe that if it isn't there yet, it probably will be in the future."

The growing popularity of rock cocaine has led to a series of police raids on what are called "rock houses" throughout the city.

These apartments or small houses often have barred windows and steel doors to impede raiders so that incriminating evidence can be destroyed before police enter. A Los Angeles woman died Dec. 13 apparently because she was too close to a diversionary explosive device used by police in raiding a rock house on West 60th Street.

According to Capt. Curt Caraway, Los Angeles Sheriff's Department narcotics bureau commander, cocaine "rocks" are made by mixing the drug with baking soda and water, then heating the mixture until it dries into a hard pellet. Only 20 to 30 percent of the mixture is cocaine, he said. Users feel they enjoy a quicker rush of euphoria smoking the pellets in a pipe than sniffing the powder, Caraway said.

Police officers and drug treatment counselors said the steel-door dispensaries are growing like a new fast-food chain. Byrd, who is attempting to treat young cocaine users with what he calls "humanistic introspection" therapy, thinks the electronic media have unconsciously advertised the drug through films and television series showing cocaine used in glamorous surroundings.

The Los Angeles police bureau that handles the city neighborhoods south of downtown reports about 150 juveniles arrested on cocaine-related charges this year, compared with 47 in 1983. Officers in the area have raided more than 150 rock houses this year. Police have reported finding as much as $35,000 in cash in a single raid.

The cocaine users treated at the Jamaa center usually are 25 to 30 years old, Byrd said. Many are buying rock cocaine with welfare money, he said. Police note that the rock houses do some of their best business on the 1st and 15th of the month, when welfare checks are mailed.

As cocaine moves into the city's poorer neighborhoods, health officials are expressing some concern about another fad among youths in more affluent neighborhoods.

A taste for Indonesian clove cigarettes, begun as a fad among surfers, has spread to high schools throughout the West Coast.

On May 10, Orange County high school student Tim Cislaw died of severe lung inflammation, which doctors now think might have been caused by a suspected toxin called eugenol in the clove cigarettes he had smoked for several months.