A deficient brain serotonin function has been demonstrated in psychiatric disorders and increases in brain serotonin ameliorate emotional and behavioural abnormalities found in depression. The uptake of the serotonin precursor tryptophan into the brain is dependent on nutrients that influence the cerebral availability of tryptophan via a change in the ratio of plasma tryptophan to the sum of the other large neutral amino acids (Trp/LNAA). In the present study, we investigated the time- and dose dependent effects of ?-lactalbumin enriched whey protein with a high tryptophan content in combination with two doses of glucose on plasma Trp/LNAA. Twelve healthy subjects participated in a double-blind cross-over study and ingested 200 ml orange juice added with glucose alone or mixed with either 5,10, and 20 g ?-lactalbumin or 20 g ?-lactalbumin hydrolysate. One half of the subjects received the orange juice with 25 g glucose, whereas the other half of the subjects received the juice with 50 g glucose. Results revealed a relationship between ?-lactalbumin dosage and increases in plasma Trp/LNAA [P<0.001]. Different doses interacted with time-dependent changes in plasma Trp/LNAA [P <0.0001] such that the most prolonged increase was found with 20 g ?-lactalbumin [P <0.0001]. Changes in plasma Trp/LNAA were not affected by glucose dose. Since increases in plasma Trp/LNAA are indicative of increased serotonin concentrations in the brain, this study shows that ingestion of 20 g ?-lactalbumin may be a suitable dietary method to investigate the effect of alterations in brain serotonin.