The aim of the study was to investigate the bioavailability of tryptophan (Trp) from a Trp-enriched peptide mixture in healthy men. A second objective was to investigate the effect of this Trp-enriched protein hydrolysate on potential parameters of serotonergic activity. serum serotonim melatonin and prolactin and urinary 5-hydroxyindol-acetic acid and 6-sulfatoxymelatonin. Glucose and insulin were measured as the placebo consisted of orange juice with added glucose. The study was a randomised, double blind, placebo-controlled, four-way crossover trial in eight healthy male subjects. Treatments consisted of a single oral load of a Trp-enriched protein hydrolysate containing 500 mg L-Trp, 500 mg L-Trp plus addition of a mixture of amino acids having the same composition as in the hydrolysate, 500 mg L-Trp, and placebo. Before and after ingestion blood and urine were collected during a period of 4 hours. The Trp-enriched protein hydrolysate increased the plasma Trp/LNAA ratio by about 70% at 60 to 90 min after ingestion. AM three Trp treatments induced a comparable plasma Trp-response demonstrating that Trp is readily available for the body independent of whether it was peptide-bonded or in free amino acid form. None of the Trp treatments affected serum serotonim prolactin, melatonin and insulin concentrations, as well as urinary 5-hydroxyindol-acetic acid and 64-sulfatoxymelatonin concentrations during the 4-hour measurement period in the morning. Copyright © 2004 by New Century Health Publishers, LLC.