Brain serotonin synthesis depends on the uptake of its precursor, tryptophan (Trp), and is correlated to the plasma ratio of Trp to large neutral amino acids (LNAA) which compete for the same transporter system in the brain. As the plasma Trp/LNAA ratio decreases when the dietary protein content exceeds 5%, we tested whether a diet containing 17% of a Trp-rich protein, namely alpha-lactalbumin (LAC), might increase the plasma Trp/LNAA ratio over a long period. Blood samples were obtained at different days (-1, 3, 6 and 9) from rats receiving either a LAC or casein (CAS) diet, and plasma amino acids and insulin concentrations were determined. The increase in plasma Trp concentration was much higher during the LAC diet (49 vs 26%; P<0.001), while the plasma LNAA concentration remained fairly constant. Consequently, the plasma Trp/LNAA ratio increased by 40% during the LAC diet while it decreased by 15% during the CAS diet (P<0.001). The above results were not related to plasma insulin concentration differences during these diets. These data suggest that a balanced diet containing a natural Trp rich-protein increases the plasma Trp/LNAA ratio over a long period, leading to a probable increase in brain serotonin activity.