The metabolism of l-tryptophan by liver cells from guinea pigs, gerbils, and sheep was studied. The rate of tryptophan oxidation was less in all three species examined than in the rat. In all three species, a much higher proportion of tryptophan carbon was metabolized through the citric acid cycle than in the rat. The accumulation of quinolinate was very low in guinea pig and sheep, and this correlated with the lack of inhibition of gluconeogenesis by tryptophan in these species. Tryptophan is a weak inhibitor of gluconeogenesis in the gerbil, and this again is consistent with a limited capacity for quinolinate formation. There was no correlation between the extent of tryptophan inhibition of gluconeogenesis and the intracellular distribution of phosphoenolpyruvate carboxykinase. Administration of tryptophan to guinea pigs in vivo had no effect on glucose turnover or on phosphoenolpyruvate carboxykinase activity.