DL-5-hydroxytryptophan, 5-HTP is a serotonin precursor. It was discovered on the rats, that the pharmacological responses and changes in brain 5-hydroxytryptamine (5-HT) concentration elicited following the administration of 5-HTP did not reflect changes in the concentration of endogenous brain 5-HT. The biochemical and pharmacological properties of the 5-HTP-induced increases in brain 5-HT concentration were distinctly different from those of increases in the endogenous content of the amine. In normal rats, administered 5-HTP, observable pharmacological responses were not elicited, despite equal or even larger increases in brain 5-HT concentration.