A biologist at Northwestern University, Ernest Moore, has stumbled upon a drug that could treat a condition that plagues more than 50 million Americans, or about one-sixth of the population.

Tinnitus, characterized by a constant ringing noise in the ears, is currently helped by a mechanical device that look similar to a hearing aide that emit sounds in an attempt to mask the inner-ear noise. Now, Mr. Moore says he may have found a drug that can prevent tinnitus from developing. He applied earlier this year to launch a clinical trial for the drug, which he researched with the help of an unorthodox subject — the tiny zebrafish.

Scientists traditionally use rats or mice for such experimental drug trials and research. But the zebrafish, no bigger than the average goldfish, is proving to be Mr. Moore’s secret weapon against tinnitus.

The physiology of a zebrafish is fairly similar to that of a human, including its ears. “These fish have hair cells, just like humans, which are receptors of sound,” Mr. Moore said. “They also have very similar auditory nerves that carry information to the brain stem and then to the brain.”

Certain characteristics of zebrafish work particularly well for research, including the transparency of the fish, which allows Mr. Moore to see immediate changes to the inner workings of his test subjects. He also can administer test drugs to the fish by simply adding doses to the water in which they swim, instead of having to inject them, as would be necessary with test rodents.

Tinnitus affects zebrafish much as it does humans, in that the constant ringing noise in the ears causes them to experience psychological pain and distress. The biologist can detect levels of these effects by watching for erratic swimming patterns. He then looks at the cells in the ears of the zebrafish to see if electrical firings have increased, which is an early sign of damage and tinnitus.

Mr. Moore said he hopes that the drugs he is developing will block electric firings in order to return the cells to their normal activity. “If the hair cell is not totally damaged but just beginning to break down, and you administer these drugs, you might be able to prevent further damage and interfere with the cells’ ability to generate tinnitus,” he said.

As powerful in-ear headphones from MP3 players and iPods increasingly become a must-have accessory for a generation of Americans, the condition is growing more widespread and worsening for the people who have it. Although scientists generally agree that prolonged exposure to loud noises is the primary cause of tinnitus, people who have never experienced such noise can be stricken as well.

“The world is a loud place and it’s getting louder every day,” the vice chairman of the ear, nose, and throat department at NewYork-Presbyterian Hospital, Dr. Samuel Selesnick, said. “We don’t have control over some of this noise, but in some cases, as in the case of headphones, we do.”

For his part, Mr. Moore suspects he developed tinnitus because of his exposure to the sounds of rifle fire and artillery explosions while he was fighting in the Gulf War. He was also an avid hunter for decades. “Ears don’t bleed from tinnitus. It’s not obvious and dramatic, like a heart attack or cancer, although it torments its sufferers,” he said.

Dr. Selesnick, who is also the president-elect of the American Neurotology Society, has seen patients who are classically trained violinists and did nothing more than spend decades sitting in orchestra pits near trumpets and trombones, as well as those who worked for years on airport tarmacs near the deafening drone of jet engines.

Tinnitus is frustrating to treat, Dr. Selesnick said. “Sometimes it’s a ringing in the ears, other times it can be a steady pulse that’s felt deeper in the head. It’s not like you can prescribe an antibiotic and make it go away.”

Also frustrating to physicians is that once damage occurs to nerves within the ears and tinnitus sets in, there is no reversing it. Unlike, say, lung tissue, which can rebound once a person stops smoking, cell tissue and nerve endings in the ears are far more delicate.