Single oral doses of L-5-hydroxytryptophan (5-HTP) were administered in combination with L-aromatic amino acid decarboxylase inhibitors. The time courses of plasma concentrations of 5-HTP, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) and the concentrations of 5-HT in blood platelets were measured. Carbidopa enhanced the rise in plasma concentrations of 5-HTP 5-15 fold and counteracted the increase in plasma 5-HIAA levels induced by 5-HTP alone. A single dose of the decarboxylase inhibitor was equipotent to 14 days' pretreatment. Plasma or platelet concentrations of 5-HT failed to reflect the metabolism of 5-HTP. The ratio of 5-HTP to carbidopa influenced the systemic bioavailability of single dose administered 5-HTP indicating dose dependent absorption kinetics. Co-administration of L-dopa with 5-HTP and decarboxylase inhibitors had no effect on gastrointestinal absorption of 5-HTP in six parkinsonian patients.