Mature sows were fed for a total of 72 lactations with diets which provided an adequate supply of energy and nutrients except for tryptophan (Trp). By supplementing a basal diet [native 1.2 g Trp/kg, equivalent to 0.8 g apparent ileal digestible (AID) Trp or 0.9 g true ileal digestible (TID) Trp] with L-Trp, five further diets (2-6) containing 1.5-4.2 g Trp/kg were formulated. The dietary Trp content had no effect on amino acid contents in milk on days 20 and 21 of lactation, but Trp in blood plasma on day 28 of lactation reflected the alimentary Trp supply with an increase from 2.74 +/- 1.14 mg/l (diet 1) to 23.91 +/- 7.53 mg/l (diet 6; p < 0.001). There were no directional differences between the diets with regard to the other amino acids. Concentrations of urea in milk and blood were higher with diet 1 (211 and 272 mg/l, respectively) than with diets 3-6 (183 and 227 mg/l, respectively). Serotonin levels in the blood serum were lower with diet 1 (304 ng/ml) than the average of diets 4-6 (540 ng/ml). This study confirms previously given recommendations for the Trp content in the diet of lactating sows, estimated by means of performance, of 1.9 g AID Trp (equivalent to 2.0 g TID Trp; approximately 2.6 g gross Trp) per kg diet.