The effect of prenatal protein malnutrition on central serotonin metabolism was assessed in 220- to 240-d-old male rats. The malnourished rats (denoted 6,25 group) were males born to dams fed a 6% casein diet during pregnancy and fostered at birth to dams fed a control (25% casein) diet. They were compared with males born to dams fed 25% casein diet. Tissue concentrations of serotonin, 5-hydroxyindoleacetic acid, 5-hydroxytryptophan, L-tryptophan and catecholamines in the hippocampal formation in the 6,25 group were similar to those of well-fed controls (25,25 group). However, a twofold greater basal serotonin efflux from hippocampal slices of 6,25 rats compared with slices from 25,25 rats was observed during a 20-min incubation period. Hippocampal [3H]paroxetine binding indicated that there was no alteration of apparent maximal binding and affinity of the serotonin transporter in the 6,25 rats. In addition, there was no difference in serotonin receptor binding in hippocampal membranes from 6,25 and 25,25 rats. The results indicate that prenatal protein malnutrition causes selective changes in central serotonin metabolism.