Picolinic acid (PiA) is an endogenous metabolite of tryptophan that has been reported to possess a wide range of physiological actions. We investigated the effects of dietary PiA on the metabolism of tryptophan to nicotinamide in growing rats. Feeding an ordinary diet containing 1% PiA to growing rats (6 wk) caused death within a few days. Toxicity of PiA was higher than that of analogs such as nicotinic acid and quinolinic acid. Feeding an ordinary diet containing 0.05% and 0.1% PiA did not elicit decreased intake of food or loss in body weight. PiA did not affect the in vitro liver activities of quinolinic acid phosphoribosyltransferase or α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSDase, a Zn-dependent enzyme). Concentrations of NAD and NADP in the liver and blood were not affected by PiA. PiA administration did not affect tryptophan metabolites such as anthranilic acid, kynurenic acid, and xanthurenic acid. However, quinolinic acid and subsequent metabolites such as nicotinamide and its catabolites were increased by administration of a diet containing 0.05% PiA but not by a 0.1% PiA diet. These results suggest that the in vivo activity of ACMSDase is controlled by the Zn level. Therefore, a small amount of PiA has a beneficial effect for conversion of tryptophan to nicotinamide, but an excessive amount of PiA can be very toxic.