Ohio must stop using a common combination of three chemicals to execute condemned inmates because they may produce excruciating pain, a state court judge there ruled Tuesday.

Then, in what legal experts said was a first, the judge instead ordered the state to start using a single large dose of barbiturate, common in animal euthanasia.

The decision is an exception to recent judicial trends in the wake of the United States Supreme Court's decision in April in Baze v. Rees, which upheld Kentucky's lethal injection protocol, similar to the one used in Ohio.

There have been five executions — two in Georgia and one each in Mississippi, South Carolina and Virginia — since Baze ended a de facto seven-month moratorium. And Texas is to resume executions on Wednesday.

The Texas Court of Criminal Appeals issued rulings Monday that rejected challenges from five death row inmates to lethal injections there, which also rely on the three-chemical cocktail. Karl Chamberlain, who raped and murdered a 30-year-old woman in 1991, is scheduled to be executed on Wednesday.

The Baze decision did not foreclose all challenges to lethal injections under the Eighth Amendment, which bars cruel and unusual punishment. But it said challengers must show a demonstrated risk of severe pain along with a feasible alternative that would significantly reduce that risk.

The Ohio judge, James M. Burge of the Lorain County Court of Common Pleas in Elyria, appeared to concede that a constitutional challenge to the Ohio protocol would fail under Baze. Judge Burge based his decision instead on an Ohio law requiring that lethal injections use ''a drug or combination of drugs of sufficient dosage to quickly and painlessly cause death.''

Baze, Judge Burge wrote, said the Constitution did not require the avoidance of all risk of pain. The Ohio law, by contrast, he said, ''demands the avoidance of any unnecessary risk of pain and, as well, any unnecessary expectation by the condemned person that his execution may be agonizing or excruciatingly painful.''

The three chemicals used in Ohio and elsewhere are a sedative, a paralyzing agent and a drug that stops the heart. If the first is administered improperly, Judge Burge wrote, the second and third chemicals can give rise to suffocation and intense pain.

Commissions appointed to study lethal injections have questioned the three-chemical combination, as have some judges, saying it is cumbersome and risky. But Judge Burge was the first judge to require a sedative-only protocol, experts in the field said.

The case was brought by Ruben O. Rivera and Ronald McCloud, who have been charged with capital crimes but have not yet been tried. The state had sought to halt the proceeding before Judge Burge as premature, but the Ohio Supreme Court declined to intercede. Mr. Rivera is charged with killing a man during a robbery. Mr. McCloud is charged with raping and killing a woman.

Elisabeth A. Semel, the director of the Death Penalty Clinic at the University of California, Berkeley, welcomed Judge Burge's ruling.

''It is likely to reduce the risk of executions that cause suffocation and excruciating pain,'' Professor Semel said.

A spokeswoman for the Ohio attorney general's office said lawyers there were reviewing the decision.

The last execution to be carried out before the de facto moratorium was in Texas, and it was the subject of controversy. The presiding judge on the Court of Criminal Appeals, Sharon Faye Keller, closed the courthouse at its regular time, 5 p.m., turning back an effort by a death row inmate to file appeal papers a few minutes later. The inmate, Michael Richard, was executed that evening.

Opponents of the death penalty in Texas said they had hoped the criticism that followed that episode would inform the court's decisions on the recent lethal injection challenges.

''Those of us who thought the court was taking a thoughtful and rigorous look at this were wrong,'' said David R. Dow of the Texas Defender Service. ''I don't think the court has shown the interest in really developing a coherent body of law, but instead reacts from one decision to the next like a pinball.''

Mr. Chamberlain, scheduled to die on Wednesday, would be the 406th prisoner to be executed in Texas since the Supreme Court reinstated the death penalty in 1976.

''Texas is still Texas,'' said Richard Dieter, executive director of the Death Penalty Information Center, which opposes capital punishment. ''They are the ones with the most dates coming up, and by the year's end I'm quite confident they will lead the country in executions.''

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