WASHINGTON — Moonlight and soft music may help, but it’s a few chemical molecules that actually raise the male response to romance. Without nitric oxide, researchers say, the mating call could go unanswered.

Scientists at Johns Hopkins University in Baltimore used electrodes to stimulate male rats and determine that penile erection is triggered by minute amounts of nitric oxide, a chemical generated by nerves. The same chemical also is found in human nerves.

Dr. Arthur Burnett, a Hopkins urologist and co-author of a report in the journal Science, said medical scientists have tried for more than a century to identify the chemical signal that sets off the cellular events in male arousal.

Burnett said the role of nitric oxide lasts only a few seconds in the complex process of erection, but the chemical is an essential trigger.

“It’s like turning on a light switch,” said Burnett. While wires, lamps and light bulbs are all needed to light up a room, he said, “it is the switch that starts the process. Nitric oxide serves that function in erection.”

Dr. Solomon Snyder of Hopkins, another co-author of the study, said the discovery of the chemical trigger for erection should have direct clinical applications within five years.

“For people who have impotence, drugs that generate nitric oxide may relieve the problem,” he said. “Ideally, you would actually apply it directly to the penis,” probably in a cream that could be absorbed.

Nitric oxide drugs may be developed even more quickly for men with the opposite problem — excessive erection. The disorder, called priapism, causes a persistent and painful engorgement of the penis. About 40 percent of all men with sickle cell anemia experience the problem at some time.

“A drug that inhibits nitric oxide should be therapeutic for these patients,” said Snyder.

The physician emphasized, however, that nitric oxide-based drugs will never relieve all types of impotence nor provide a complete solution to priapism.

Burnett said that nitric oxide is not to be confused with nitrous oxide, the gas used as anesthetic.

“Nitrous oxide is laughing gas and is very stable,” he said. “Nitric oxide is an almost noxious gas that has a very potent biological function and is produced in certain cells.”

A male erection in most mammals occurs when smooth muscles in the penis are relaxed and blood vessels dilate. This allows blood to pool and engorge the tissue.