The acute pharmacological actions of methylphenidate, a potent psychotogenic stimulant, were studied by measuring plasma homovanillic acid (HVA) and serum growth hormone (GH) as indices of presynaptic and postsynaptic functions, respectively. Twenty psychiatric inpatients participated in a double-blind, placebo-randomized study of methylphenidate infusion. Multiple samples of plasma HVA and GH were measured before and up to 24 hours after both methylphenidate and placebo infusions. There was a robust and significant GH response during the 2-hour postmethylphenidate period when compared to placebo. The peak response was significantly greater in male patients than in female patients. Although there were no changes in plasma HVA during the 2 hours following the infusion, there was a significant decrease in HVA 24 hours after methylphenidate as compared to preinfusion baseline values. Plasma HVA values also tended to be lower 24 hours after methylphenidate when compared to the corresponding postplacebo value. Schizophrenic patients did not differ significantly from nonschizophrenic patients in their physiological or biochemical responses.