Trazodone, 2-[3-[4-(m-chlorophenyl)-1-piperazinyl]propyl]-s- triazolo[4,3-a]pyridin-3(2H)one, was evaluated as an inhibitor of uptake into serotonin neurons in vivo in the brains of mice and rats by determining its ability to antagonize the depletion of brain serotonin by p-chloroamphetamine. In mice, trazodone was inactive under conditions in which many antidepressant drugs and other inhibitors of uptake are potent antagonists of the depletion of serotonin in brain induced by p-chloroamphetamine. Weak inhibition of uptake into serotonin-containing neurons in brain in vivo was demonstrated early after the injection of trazodone in rats, especially when the dose of p-chloroamphetamine was reduced to facilitate competitive inhibition of its effects. However, the effects of trazodone were short-lasting. Trazodone did not potentiate the elevation of serum corticosterone by L-5-hydroxytryptophan, in contrast to the effect of fluoxetine, a potent and selective inhibitor of the uptake of serotonin. Instead, trazodone antagonized a response mediated by a serotonin receptor, i.e. elevation of serum corticosterone by a serotonin agonist, quipazine, in rats. Trazodone also antagonized the serotonin-induced contraction of the rat jugular vein in vitro (a response mediated by 5-HT2 receptors), the pA2 being 8.79. These findings agree with previous reports that trazodone is a potent antagonist of serotonergic function. These data, together with earlier evidence, suggest it is unlikely that the inhibition of uptake of serotonin contributes to the clinical antidepressant effects of trazodone.