The pharmacokinetics of 5-hydroxytryptophan were investigated in relation to intravenous administration of single doses of the amino acids (0.2 mg/kg b.wt.) to five patients pretreated for one week with carbidopa, an L-aromatic amino acid decarboxylase inhibitor. The plasma concentration/time lapse of 5-hydroxytryptophan exhibited bi-exponential disposition characteristics and the data obtained could be closely fitted to an open two compartment pharmacokinetic model with elimination taking place from the central compartment. The apparent composite 1. order rate constants alpha and beta were 1.433 hr-1 +/- 0.177 S.D. and 0.113 hr-1 +/- 0.012 hr-1 S.D., respectively. The biological half-life of 5-hydroxytryptophan under the experimental conditions was thus about 6 hours. The apparent volume of the central compartment, Vc, was found to be 0.336 1 kg-1 +/- 0.059 S.D. and the plasma clearance 0.105 1 hr-1 kg-1 +/- 0.015 S.D. The derived pharmacokinetic constants were used for doses regimen calculations and predetermination of the plasma concentration/time lapse, during continuous intravenous infusion therapy. Measured plasma concentrations of 5-hydroxytryptophan during infusion deviated by less than 10 per cent from the predicted values. However, the infusions had to be stopped in four of the experiments because the patients became nauseated and vomited after total doses of only 36--128 mg.