The immediate precursor in the serotonin synthetic route, 5-hydroxytryptophan (5-HTP), labeled with 11C in the beta position, has become available for studies using positron emission tomography (PET) to examine serotonin formation in human brain. Normalized uptake and intracerebral utilization of tracer amounts of [beta-11C]5-HTP were studied twice in six healthy male volunteers, three of them before and after pharmacological pretreatments. The kinetic model defines regional utilization as the relative regional radioactivity accumulation rate. Repeat studies showed good reproducibility. Pretreatments with benserazide, p-chlorophenylalanine (PCPA), and unlabeled 5-HTP all significantly increased uptake of [beta-11C]5-HTP. The utilization rates in both striatal and frontal cortex were higher than those in the surrounding brain, indicating that PET studies using [beta-11C]5-HTP as a ligand quantitate selective processes in the utilization of 5-HTP. We tentatively interpret uptake and utilization as a measure of brain serotonin turnover, the selectivity of which was shown by pharmacological interventions in vivo.