Plasma membrane vesicle preparations from mouse cerebral cortex actively accumulated [3H]serotonin upon the imposition of a K+ gradient (in greater than out), a Na+ gradient (out greater than in), and the presence of external Cl-. Maximal stimulation of uptake by internal K+ occurred at 15 mM and half-maximal stimulation at 2 mM. Internal K+ did not enhance uptake merely via generation of a membrane potential because simultaneous parallel increases in internal and external K+ concentration also stimulated uptake. External Cl- increased serotonin uptake with a Km of 18 mM and a Hill number of 1.0, suggesting a requirement for one chloride ion for transport. Uptake could not be driven by internal H+ instead of K+. Estimation of the membrane potential by the distribution of triphenylmethylphosphonium ion showed a modest effect of valinomycin (1-20 microM) in increasing the potential from -19 to -31 mV accompanied by an increase in serotonin uptake. Proton ionophores prevented this effect of valinomycin and, by themselves, reduced the potential to -6 mV, but did not affect serotonin transport. A model is proposed for serotonin transport in brain plasma membrane vesicles that is similar to the model for porcine blood platelet vesicles as far as electroneutrality and stimulation by K+, Na+, and Cl- are concerned, but that is different in substitution of internal H+ for K+.