THE EXECUTION of Charles Brooks Jr. in Texas last week raises two separate, yet equally disturbing questions whose implications go well beyond the case in question. Brooks was convicted of shooting a Fort Worth car mechanic and sentenced to death, even though his co-defendant received a prison term of 40 years and it remains unclear which of the two pulled the trigger. In addition, the way Brooks was put to death--by injection--has caused a careful scrutiny of medical ethics by those who believe a doctor should preserve and not take lives.

Six years ago, Brooks and Woody Loudres allegedly murdered the mechanic, David Gregory. Tried together, neither would admit who did the shooting, and both were found guilty. But Loudres, resorting to the appeals process, agreed to plead guilty in return for a reduced sentence. Brooks, who maintained his innocence all along, got death.

The idea that two men should be tried for exactly the same crime without any evidence to prove who did what and get dramatically different sentences seems absurd. When one of the sentences meted out is death, the absurdity becomes a sickening disgrace. You don't have to be an opponent of capital punishment to see the crime in the execution of Brooks, only someone who respects the law. The law says that guilt must be proved "beyond reasonable doubt." But no one knows who pulled the trigger.

The other issue in the Brooks fiasco--the questions raised by death by injection--itself will surely prove the starting point for a long debate. One problem is that the method has simply not been adequately tested. Thought to be quick and painless, the injection technique could also cause a bloodbath. As one doctor explained: "Sometimes it's tough as hell to find a vein. If you miss, the pain can be excruciating. And besides, even if you do get the vein, no one is really sure the convict doesn't suffer for a few minutes while the drug is being administered."

Even more complex a question is what role--if any--doctors should play in administering the injection. In 1980, the American Medical Association (AMA) adopted a policy that "a physician, as a member of a profession dedicated to preserving life when there is hope of doing so, should not be a participant in a legally authorized execution." The Texas Medical Association has reiterated this position.

But although Dr. Ralph Gray, the medical director of the Texas Department of Correction, neither inserted the needle into Brook's arm nor fed the drugs into the intravenous tube, he examined Brooks to make sure his veins were large enough to accept the needle and prescribed and mixed the dosage of chemicals that killed him.

Gray and other doctors claim the AMA policy applies only to direct participation in an execution. But some are not so sure.

"I think the AMA understood what was being driven at, and I think the state association did, too," said Dr. Willian Curran, professor of legal medicine at the Medical School. "It was never just the physical act of inserting the needle or the substance that they were thinking of."

In time, it may be found that death by injection is the best way to carry out capital punishment. Then doctors will have to take up the weightily question of whether to put aside their aversion to the death penalty in order to give a convict the most humane treatment possible under the law. But for now, it seems an investigation of Gray's conduct is in order. The AMA has taken a stand against participating in executions. All doctors should be held so it.