The basal activity of tryptophan 2,3-dioxygenase (EC-1.13.11.11) in primary cultured rat hepatocytes decreased during culture, but addition of either tryptophan (2.5 x 10(-3) M) or dexamethasone (1 x 10(-6) M) could prevent the decrease. Addition of both compounds caused severalfold induction of activity. Glucagon (1 x 10(-8) M) alone did not induce the activity, but its inductive effect in combination with tryptophan was similar to that of tryptophan plus dexamethasone. The effect of glucagon was additive with those of tryptophan and dexamethasone and hence the highest induction (7-fold) was achieved by addition of all three inducers. Glucagon could be replaced by dibutyryl cyclic AMP (1 x 10(-5) M). Insulin (1 x 10(-8) M) inhibited the inductions by glucagon and dexamethasone, but not that by tryptophan. Cycloheximide inhibited the inductions by all three inducers, but actinomycin D inhibited only the induction by dexamethasone. These results suggest that the three compounds have different mechanisms of induction of tryptophan oxygenase activity: tryptophan prevents enzyme inactivation, dexamethasone may stimulate enzyme synthesis at the level of transcription, and glucagon may enhance the synthesis at the translational level.