The in vivo assay of tryptophan pyrrolase gives best results when [ring-2-14C] tryptophan is used and rates of labelled CO2 production are measured at short times after the labelled tryptophan injection. Using this technique in conjunction with in vitro measurements, we find that both tryptophan pyrrolase activity and the liver tryptophan concentration are important factors in controlling the rate of tryptophan breakdown by pyrrolase. Tryptophan in the liver has a very high turnover rate, and the enzyme activity and the liver tryptophan concentration interact. Thus a high liver tryptophan increases the enzyme to speed tryptophan breakdown, while a high pyrrolase activity may tend to lower liver tryptophan, thus tending to decrease both the rate of tryptophan catabolism and possibly the enzyme activity.