1-(1-Naphthyl)piperazine (1-NP) had high affinity for tritiated serotonin, tritiated LSD (lysergic acid diethylamide) and tritiated spiperone binding sites in rat brain cortex in vitro. 1-NP at doses of 3-30 mg/kg i.p. decreased 5-hydroxyindoleacetic acid (5-HIAA) concentration in whole brain of rats in vivo. The 30 mg/kg dose caused a significant increase in serum corticosterone concentration. At doses of 3-30 mg/kg i.p., 1-NP reduced the accumulation of 5-hydroxytryptophan following decarboxylase inhibition by NSD 1015 in rat hypothalamus and striatum. Reduced serotonin turnover and elevated serum corticosterone concentrations are interpreted as evidence of central serotonin receptor activation by compounds of this structural class. 1-NP has previously been reported to antagonize vascular serotonin receptors, suggesting that it, like 1-(m-trifluoromethylphenyl)piperazine, behaves as an antagonist at peripheral (vascular) serotonin receptors despite being an agonist at central serotonin receptors.