d-Fenfluramine is an anorectic drug believed to act by enhancement on serotonergic function in the brain. d-Fenfluramine (or the racemate) releases serotonin through a carrier-dependent mechanism, and serotonin release is the mechanism usually thought to produce its serotonergic effects. However, d-fenfluramine also inhibits serotonin uptake in vitro, and serotonin uptake inhibition is sometimes suggested to contribute to its mechanism of anorectic activity. Neurochemical experiments were done to examine serotonin release and serotonin uptake inhibition as mechanisms of action of d-fenfluramine in rats and to compare d-fenfluramine to fluoxetine, a serotonin uptake inhibitor. d-Fenfluramine decreased serotonin concentration in rat brain as early as 1 hr; at 1 hr 5-hydroxyindoleacetic acid (5HIAA) concentration was slightly increased, but at later times 5HIAA was also decreased. Fluoxetine, in contrast, did not change serotonin concentration in whole brain but decreased 5HIAA concentration at all time points. At all time intervals studied, the 5HIAA/serotonin ratio was increased by d-fenfluramine (and by Ro 4-1284, a nonspecific serotonin releaser) but was decreased by fluoxetine, a serotonin uptake inhibitor. No decrease in 5HIAA concentration or in the 5HIAA/serotonin ratio was apparent at any time or after any dose of d-fenfluramine studied. The possibility that doses of d-fenfluramine below those needed for serotonin release might inhibit serotonin uptake was tested by determining whether d-fenfluramine could block the acute depletion of brain serotonin by p-chloroamphetamine, or the long-term neurotoxic effect of p-chloroamphetamine on brain serotonin neurons.(ABSTRACT TRUNCATED AT 250 WORDS)