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In his library research, Dr. Thistle noted that MTBE remains liquid in the body. He first tested the effects of MTBE by dissolving gallstones in test tubes. Then he implanted multiple human gallstones in a dog's gallbladder and tested the speed with which the ether solvent dissolved them. He found that not only did MTBE act more rapidly than diethyl ether and another solvent, mono-octanoin, used to dissolve gallstones in the bile ducts, but it did so without irritating the lining of the animals' gallbladders.

In recent years radiologists have often used skinny needles to explore various areas of the body for diagnostic and therapeutic purposes.

So Dr. Thistle enlisted the aid of Dr. Gerald R. May, a radiologist at Mayo Clinic, to do his first human experiment in March 1983. The patient was a 64-year-old woman who refused surgery. With the MTBE therapy her gallstones dissolved in seven hours. Since then Mayo Clinic officials have approved the therapy as an investigational procedure.

In years past most gallstones were diagnosed on X-rays after an individual swallowed Telepaque pills, which contain a chemical that is opaque to X-rays. The radio-opaque Telepaque serves as a visual contrast to the gallstones, many of which could not otherwise be seen because they are radiolucent. Gallstones can also be detected with ultrasound tests.

When a person with gallstones is referred to Dr. Thistle, the doctor first determines if the gallstones are causing symptoms and, if so, how severe they are.

Once satisfied that the gallstones are the source of the symptoms, the doctors determine if they are cholesterol stones. For that purpose conventional gallbladder X-rays are used.

Appearance of the gallbladder as a white area on the X-rays shows that it is functioning normally and that there is no blockage of the cystic duct. The doctors also study the X-rays for evidence of calcium deposits, which, if present in the stones, would disqualify the patient from the therapy because that would sharply reduce the chances of complete dissolution.

The night before the ether solvent procedure the patient takes Telepaque pills. In the morning a radiologist uses a fluoroscope, a machine that allows a doctor to produce a continuous series of X-rays much like a movie, to make certain no stone has escaped to block the ducts and to locate the gallbladder.

As the patient lies on his back, xylocaine, a local anesthetic, is injected in the skin over the area where the tube will be inserted and fentanyl, a strong narcotic, is injected into a vein. Then the doctors, guided by the fluoroscope, thrust a long, skinny needle through the skin and liver into the gallbladder. They run a guide wire through the hollow needle and remove the needle.

The purpose of these steps is to create a thin pathway so a tube 1.7 millimeters in diameter, or about the thickness of a strand of spaghetti, can be threaded along the guide wire into the gallbladder.

Then Dr. Thistle uses a syringe to withdraw the bile and begins to replace it with MTBE, being careful not to let bile leak or the ether solvent spill into the liver or peritoneal cavity; the chemical could cause serious complications.

Stirring MTBE increases its dissolving powers. The doctors achieve that effect with a small pump.

The time it takes to dissolve all the gallstones depends on their number - there can be scores - and their size - larger ones take longer. The range is from seven to eighteen hours, and in that time the patient can read or talk. The treatment can be spread over two or three days, allowing the patient and the doctor to rest after an eight-hour session.

When the doctors can no longer see gallstones on the fluoroscope, another radiologist performs an ultrasound test, which Dr. Thistle said he has found more effective than X-ray in detecting tiny stones less than two millimeters in diameter.

The following day the patient goes home. Usually he is able to return to work a few days later. The entire hospital stay is about four days, in contrast to a week or so for a standard gallbladder operation.

So far the costs of the tests and the procedure match those of the conventional gallbladder surgery.

Dr. Thistle said he considered it unlikely that toxic effects would show up long after MTBE therapy because the body rids itself of the substance quickly through the breath. Of the side effects detected to date, naeaea and vomiting have been the leading ones, and three patients have reported feeling sleepy.

A Although Dr. Thistle expressed doubts that the MTBE therapy would be useful for every patient with cholesterol gallstones, he said he thought it might eventually spare thousands of people the risks of major surgery.

Meanwhile, Dr. Thistle stressed, ''surgery is an effective and safe treatment for'' otherwise healthy people who have gallstones. In time, he said, his and other teams would try the more difficult task of dissolving calcified gallstones.

If these measures succeed, they could have an important impact because cholesterol stones account for about 80 percent of all gallstones. The remaining 20 percent are comprised of bile pigments, which are not dissolved by either MTBE or the chenodiol pills.

It is not clear how often cholesterol gallstones will recur after the ether solvent therapy, in large part because there is such a wide variation in the incidence of gallstones among racial and national groups.

Almost 70 percent of the women in some Indian tribes get gallstones before they are 30 years old, whereas only 10 percent of black American women have gallstones. Gallstones are almost unknown among the Masai people in East Africa, and are more common among Swedes and Finns than among Americans.

Gallstones have recurred in one patient in the Mayo Clinic trial of MTBE, and that patient then had standard gallbladder surgery.