Doses of 5.0, 7.5 and 10.0 g L-tryptophan, the amino acid precursor of serotonin, or saline alone were administered by IV infusion to a group of 11 healthy male subjects, and both hormonal and behavioral responses were monitored. Significant increases were observed in plasma concentrations of growth hormone and prolactin after all three doses of L-tryptophan, but not after saline infusion. No alterations in cortisol or thryotropin were noted at any level. Examination of behavioral effects of L-tryptophan revealed a dose-dependent impairment in performance on the symbol copying test. In addition, L-tryptophan produced significant effects on mental and physical sedation, but did not alter subjective ratings of tranquilization. In agreement with some prior reports, these observations support the ability of L-tryptophan, when administered IV in high doses, to produce pronounced effects on the central nervous system in humans, and suggest the potential utility of this paradigm as a neuroendocrine challenge test.