The plaintiffs in a cigarette product-safety trial in a state court here abruptly dismissed two convenience stores as defendants today, raising the possibility that the trial will be aborted and the case moved to federal court.

Melvin M. Belli, the lawyer for Elayne Galbraith, widow of John M. Galbraith, and her three adult children, announced the dismissal to Superior Court Judge Bruce W. Dodds shortly before opening statements were made to the jury.

The decision whether to transfer the case to federal court is up to R. J. Reynolds Tobacco Co., the sole remaining defendant. Dodds told Reynolds' counsel Thomas E. Workman Jr. that he wants a decision Monday morning, before the jury starts to hear evidence in a trial that was expected to last six to eight weeks.

Belli's legal justification for having named the stores as defendants in the first place, in 1978, was that they had sold countless packs of cigarettes to Galbraith, a heavy smoker who died in 1982, shortly before his 70th birthday.

L. Donald Boden, the stores' attorney, told reporters that Workman had suggested to the judge in chambers that Belli had named the markets as defendants in 1978 in an effort to avoid the federal jurisdiction he feared might be unfavorable to his wrongful-death case.

Belli, asked why he hadn't dismissed the markets earlier, said the environment in federal court recently has improved.

The generally predictable opening statements were marked by slips and errors, and were called "very disappointing" by the judge because, he told Belli and Workman outside the presence of the jury, both lawyers had "disobeyed" his oft-repeated order not to argue the case.

Belli, 78, told reporters that his opening statement was "the worst I've ever made," partly because the judge had ordered him not to quote from the annual reports on smoking issued by the U.S. Surgeon General. When he began what he termed a "paraphrase" of part of one of the reports, Workman arose to accuse him of "misconduct." During Workman's opening statement, however, the same charge was made against him by Paul M. Monzione, Belli's associate.

Late in the day, an emergency request by Belli to stay the trial because of Dodds' ruling on the Surgeon General's reports was granted by the Court of Appeal in Ventura. Reynolds said it would file a brief by tonight. The stay could be lifted, possibly tomorrow, or could remain in effect pending a hearing.

Basically, Belli told the jury that his evidence would incriminate cigarettes in John Galbraith's death, while Workman told the jury that his evidence would exonerate smoking.

Regardless of the ultimate verdict, the case, although the first to go to trial in a new series of smokers' litigation, may have no large national impact. One reason is that Belli has done very little pretrial discovery to find out what Reynolds knew, and when, about fundamental issues, such as the health risks of smoking.

By contrast, plaintiffs' lawyers in a pending federal case in New Jersey have gathered hundreds of thousands of documents that the defendant tobacco companies themselves have said carry a "danger. . . of embarrassment, oppression and apparent incrimination" if "taken out of context."

Belli claimed that Galbraith's heavy smoking, over nearly five decades, gave him emphysema and lung cancer, and that the radiation treatments for the cancer aggravated his already weakened condition, leading to death from the immediate causes listed on the death certificate: heart disease and pulmonary fibrosis, a severe scarring of the lungs.

"The great majority of those who smoke get cancer," Belli claimed to the jury. Workman cited health data showing that "more than 90 pecent of smokers do not get cancer."

Workman, in a slip that appeared to startle other defense counsel and the judge, called nicotine an addictive substance.

His main claims were that "there is no evidence" that smoking played any role in the death of Galbraith, a man beset by major illnesses throughout his life, but a man who nonetheless far outlived his non-smoking parents and two brothers.

Belli contended that Galbraith was addicted to smoking, eliminating his freedom of choice. Workman countered that Galbraith had stopped smoking -- once for eight months -- when he wanted to do so.

Belli said that Reynolds had not warned Galbraith, who sold cigarettes himself in a 7-Eleven store he operated, of the hazards of smoking. Workman said that physicians and family members had warned Galbraith repeatedly -- and that the thousands of packs of cigarettes he sold bore the Surgeon General's warning.

Belli said his experts will testify that Galbraith had the particular type of lung cancer associated with smoking. Workman said his experts will testify that the cancer was viral-induced.

Workman also said that his evidence will show that Galbraith was an alcoholic who lived on a diet consisting of "mostly junk food" and of greasy foods, and that he constantly blamed everyone but himself for his troubles.