1-(m-Trifluoromethylphenyl)-4-(p-aminophenylethyl)piperazine (LY 156163), reported previously to have selective affinity for the 5-HT1A subtype of serotonin receptor in vitro, was studied at doses of 1.25 to 20 mg/kg i.p. in rats to determine if it had properties characteristic of centrally acting serotonin agonists. LY 165163 decreased whole brain concentrations of the serotonin metabolite, 5-hydroxyindoleacetic acid, but not of serotonin itself, decreased the rate of accumulation of 5-hydroxyindoleacetic acid after probenecid administration to block its efflux from brain, decreased the rate of decline in serotonin concentration after inhibition of serotonin synthesis with alpha-propyldopacetamide and decreased the accumulation of 5-hydroxytryptophan after decarboxylase inhibition by m-hydroxybenzylhydrazine. LY 165163 also decreased 5-hydroxyindoleacetic acid concentrations in two specific brain regions, striatum and hypothalamus. Serum concentrations of corticosterone and prolactin were increased by doses of LY 165163 that reduced serotonin turnover. These effects are all consistent with evidence from other studies that LY 165163 is a centrally acting serotonin agonist. LY 165163 also increased the concentrations of two dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, measured in whole brain as well as in striatum and hypothalamus, but did not alter dopamine concentration. The accumulation of dopa after decarboxylase inhibition was accelerated by LY 165163. The increases in hormone concentrations in serum and of dopamine metabolite concentrations in brain were not antagonized by pretreatment with metergoline, a serotonin antagonist. The mechanisms of those effects require further study.