The kinetics of l-5-hydroxytryptophan (5-HTP) were studied in five volunteers after intravenous and oral administration of 5-HTP following pretreatment with carbidopa. In addition, the effect of pretreatment with carbidopa on metabolism and disposition of 5-HTP was studied in eight subjects. The kinetics of 5-HTP following a 20-minute linear infusion are adequately described by a biexponential function. The biological half-life of 5-HTP ranged from 2.2 to 7.4 hours, and the plasma clearance ranged from 0.10 to 0.23 1/kg/hour. The bioavailability of 5-HTP after oral administration in combination with carbidopa was calculated as 48% +/- 15 (mean +/- SD). The plasma concentrations of 5-HTP observed in this study displayed an unusual double peak in most subjects after oral administration. Pretreatment with carbidopa caused a significant increase in the extent of absorption of unchanged 5-HTP, and a significant reduction in the area under the plasma concentration-time curves of 5-hydroxyindoleacetic acid. Gastrointestinal side effects appeared to be related to the 5-HTP plasma concentration.