Catabolism of Trp and Lys produces alpha-ketoadipic acid as an intermediary metabolite. An alternate pathway of Trp turnover leads to NAD synthesis. We hypothesized that excess Lys might improve the conversion of Trp to niacin by causing a buildup of alpha-ketoadipic acid, thereby endproduct inhibiting the main Trp catabolic pathway and resulting in more niacin synthesis from Trp. Six bioassays were carried out in which 12 to 20 chicks were fed each experimental diet from d 8 to d 20 or 21 posthatching. The basal diet (4 mg/kg of bioavailable niacin) used for all assays was a semipurified corn gluten meal diet fortified with crystalline amino acids to 22.5% CP and 0.96% true digestible Lys. Assay 1 through 3 established the requirements for digestible Trp (0.16%) and bioavailable niacin (19.5 mg/kg) and showed that 0.96% digestible Lys was adequate for chick growth in the presence of adequate Trp and niacin. The fourth assay was done to determine the effect of 1% Lys (1.25% food-grade L-Lys x HCl) on niacin utilization. Excess Lys improved (P < 0.01) weight gain of niacin-deficient chicks. The fifth assay showed that 1% excess food-grade Lys improved weight gain in niacin-deficient (4 mg/kg) chicks but depressed weight gain in niacin-adequate (24 mg/kg) chicks (niacin x Lys interaction, P < 0.01). In assay 6, chicks fed 6 mg/kg of niacin gained faster (P < 0.01) than control chicks, but neither quinolinic acid (100 mg/kg) nor picolinic acid (4,200 mg/kg) elicited a response. These results suggest that excess Lys leads to an accumulation of alpha-ketoadipic acid, which causes endproduct inhibition of the main Trp catabolic pathway to CO(2), therefore increasing flux of 2-amino-3-carboxymuconate semialdehyde to NAD.