The activity of 5-hydroxytryptamine (serotonin; 5-HT) in the central nervous system modulates sleep, the perception of pain and other functions of the body which might possibly relate to mechanisms of general anesthetic action. While administration of anesthetics has inconsistent effects on the content of 5-HT in brain, in vivo, accumulated data suggest that anesthetic drugs alter 5-HT homeostasis in the central nervous system. In an effort to identify one possible site of anesthetic action, the effect of halothane on the uptake of 5-HT was studied in synaptosomes isolated from the brain of rat. Established techniques were used to prepare the synaptosomal fractions and measure high affinity transport of radiolabelled 5-HT. Halothane inhibited synaptosomal accumulation of 5HT in a concentration-dependent manner, but had little effect on the passive or spontaneous release of the accumulated 5-HT. Rates of uptake of 5HT were inhibited by 43% in the presence of 1 mM halothane and by 75% of control in the presence of 5 mM halothane; the apparent I50 for halothane was 1.0 +/- 0.1 mM and Lineweaver-Burk analysis indicated the inhibition to be competitive at concentrations around the I50.