Intravenous injections of several monoamineprecursors and apomorphine were infused into 4 rhesus monkeys. Both L-dopa (15–200 mg/kg) and 5-HTP (20–60 mg/kg) induced an acute elevation in plasma growth hormone (GH) concentrations. 5-HTP induced behavioral and EEG slow wave sleep. L-tryptophan (50–100 mg/kg) induced a delayed and smaller elevation. Apomorphine (50-75 µg) and saline infusion did not induce GH release, behavioral sleep or a change in the EEG activity.