And that, Dr. Volkow told her audience with a small smile, is all about the dopamine.

She knows a little about dopamine firsthand: She is a dedicated runner and a helpless pawn in face of dark chocolate. Her most significant long-term addiction, though, has been to the science of scanning the brain with techniques that expose its workings like a map, a passion she has pursued like a guided missile since medical school.

That was in Mexico, where Europe’s 20th-century upheavals had tossed both her parents: Her mother fled Franco’s Spain, while her father, the son of Leon Trotsky’s elder daughter, joined his grandfather in exile as an orphaned teenager. Dr. Volkow and her three sisters grew up in the house in Mexico City where Trotsky was murdered in 1940, giving tours of the premises on weekends.

In medical school she read an article in Scientific American about one of the first American positron emission tomography scanners, able to photograph not only the brain’s structures but also its invisible processes. She never looked back.

“It blew my mind,” she said. After a residency in psychiatry at New York University, chosen because it owned that PET scanner, she took a job in Houston, then transplanted her research to Brookhaven National Labs on Long Island, home of groundbreaking research into dopamine and PET scanning. She and her husband, Stephen Adler, a physicist with the National Cancer Institute, now live in Bethesda, Md.

Dr. Volkow’s research career, still based at Brookhaven, has been notable for its “brilliant science,” said Don C. Des Jarlais, an expert in drug addiction who directs the Baron Edmond de Rothschild Chemical Dependency Institute at Beth Israel Medical Center in New York. Dr. Des Jarlais cited her recent widely reported study showing that cellphones alter brain metabolism as a typical example of her unusually creative scientific thinking.

A Merging of Missions

Her days now veer from reviewing raw laboratory data with her research colleagues to leading the back-to-back meetings of a government functionary, but the two roles are joined by the mantra of her time at the institute: Policy should be grounded in valid science.One recent decision in the upper echelons of the National Institutes of Health reflects a similar conclusion: The drug abuse institute and the National Institute on Alcohol Abuse and Alcoholism are on track to be merged into a joint institute on addiction still in the planning stages.

National Institutes of Health watchers have already started a body count. “It will be a big loss that Nora Volkow, current N.I.D.A. director, cannot possibly be selected to head a new institute,” wrote one anonymous blogger on the Scientopia Web site. “This would be too much like N.I.D.A. ‘winning.’ ”