Tryptophan administration increases brain tryptophan and brain serotonin levels in rats; a single intraperitoneal injection of p-chloroamphetamine produces a postural myoclonic syndrome in rats that depends on presynaptic serotonin release. In order to test the behavioral consequences of tryptophan administration, 10 mg/kg of p-chloroamphetamine was given to separate groups of rats pretreated with either 50 mg/kg of tryptophan, a suspension of neutral amino acids with tryptophan excluded or 0.9% saline. The postural syndrome was graded visually on a 0–20 point scale and the animals were killed 1–2 hr after p-chloroamphetamine injections. Tryptophan administration increased whole brain tryptophan levels 100% ( P< 0.001), increased brain and spinal cord indoleamine levels 66% ( P< 0.001), and enhanced p-chloroamphetamine-induced myoclonic behavior 34% ( P< 0.01) compared to saline controls. In separate experiments, spinal cords were transected in 9 rats and intercollicular decerebrations made in 5. Subsequent injections of 10 mg/kg of f-chloroamphetamine produced myoclonus in both groups, although other features of the postural syndrome did not occur in most decerebrate animals. It is concluded from these experiments that tryptophan-induced increases in brain and spinal cord serotonin content enhance behaviors that depend on serotonin release, and that the myoclonus induced by p-chloroamphetamine is not abolished by acute intercollicular decerebration or spinal cord transection.