In view of the abundant anatomical and functional interactions between serotonin and dopamine systems, this study examined the effect of the serotonin agonist, m-chlorophenylpiperazine (mCPP) on plasma concentrations of the dopamine metabolite, homovanillic acid. Plasma prolactin levels, body temperature, and mCPP blood level were also measured. mCPP (0.35 mg/kg) and placebo were administered orally to 10 healthy men in a randomized double-blind design. Variables were measured for 210 min after administration of capsules. mCPP raised prolactin and temperature as compared to placebo, but did not affect plasma homovanillic acid concentrations. Results suggest that mCPP does not alter dopamine function.