We examined the behavioral effects of the dietary constituents tryptophan and tyrosine on human mood, sensorimotor performance and pain sensitivity. Tryptophan and tyrosine are neurotransmitter precursors present in varying amount in protein-containing foods. Tryptophan (50 mg/kg) increased subjective drowsiness and fatigue but unlike many hypnotics did not impair sensorimotor performance. Tryptophan also decreased human pain sensitivity in a manner that was more specific than certain analgesic drugs.