California environmentalists have begun to warn that pristine stretches of the Santa Barbara area coastline may be seriously harmed as oil companies continue to uncover what may be one of the largest oil fields ever found in the United States.

Following reports of two separate giant oil discoveries by groups headed by Chevron U.S.A. Inc. and Phillips Petroleum Co. and by Texaco U.S.A., the California Coastal Commission Wednesday rejected a request from the Union Oil Co. to test drill another expected giant oil field in the federally protected eastern end of the Santa Barbara channel.

The potential size of the new discoveries near the western end of the channel is so great that environmentalists say they fear irresistible federal and corporate pressure to open offshore areas further up the coast to oil drilling, particularly since California's Democratic governor, Edmund G. (Jerry) Brown Jr., an active opponent of much offshore drilling, is about to be succeeded by Republican George Deukmejian.

"Nothing justifies in our opinion any further driving north to satisfy a gluttonous appetite for energy," said Sierra Club vice president Michele Perrault. In particular, she mentioned the danger to sea otters in a region that federal officials have targeted for leasing.

The waters off the Santa Barbara coast have attracted oil explorers ever since petroleum was found seeping naturally out of the seabed a century ago. But the major discoveries reported during the last four weeks in the Santa Maria Basin, about 40 miles west of President Reagan's mountaintop ranch, have inspired visions of an oil pool beneath the sea bottom rivaling the one found beneath Alaska's Prudhoe Bay, the largest in U.S. history with an estimated 10 billion barrels.

So far, the oil companies have confined themselves to much lower estimates, about 300 million barrels for a field being tested by Chevron, Phillips and other companies 15 miles west of Point Concepcion and about 100 million barrels for a discovery confirmed this week by a group including Texaco and the Pennzoil Co. in the same area. "It's just too soon to tell exactly what is there," said Chevron spokesman Dale Basye. But several companies are making plans for more offshore oil rigs and drilling ships, which carry the risk of future oil spills.

"It's absolutely predicted that a certain number of spills will occur in the lifetime of those fields," said Ruthann Corwin, a marine environmental planner who has participated in state and Santa Barbara County studies of the area. She said the rocky coast north of Point Concepcion is practically untouched except for a few occasional skin divers; it harbors unique species of barnacles and abalone. Sea otters, now rebounding after being decimated by hunters, are also expected to move into the area from the north.

Michael Fischer, executive director of the coastal commission, said the oil discoveries have drawn so much industry interest he expects his staff will begin considering long-range limits on oil drilling. In the key area from San Luis Obispo down to just northwest of Los Angeles, he said, "We have 20 offshore platforms drawing oil now, and there are prospects now of perhaps doubling that number." The most recent tests have drawn particular interest because the oil found appears to be low in sulfur and of a light variety easy to refine into gasoline.

Fischer said he saw no chance, however, that California's offshore waters will grow a virtual forest of oil rigs, as has happened in some parts of the Gulf of Mexico. He said he does not plan to oppose oil drilling in state-owned waters three miles from shore near the Santa Maria Basin finds, despite objections by environmentalists.

The commission rejected Union Oil's request to drill two exploratory wells near Anacapa Island because the area is within the Channel Islands National Marine Sanctuaries, a haven for brown pelicans and other marine life. This week, Exxon Co. U.S.A. announced it was seeking permission to expand its offshore drilling at a site about halfway between the Union Oil and Chevron-Texaco areas. The plan calls for two to four more platforms to help take out an estimated 400 million barrels of oil.