The developmental change in gene expression of tryptophan 2,3-dioxygenase (EC 1.13.11.11) in rat liver was studied by dot-blot hybridization with cDNA of the enzyme as a probe. The mRNA of tryptophan oxygenase is not expressed in fetal liver, but is expressed very slightly 1 day after birth. Its expression increases first gradually until 12 days after birth and then rapidly, and reaches the adult level about 22 days after birth. On the other hand, mRNA of albumin in the liver, measured with its cDNA, increases rapidly in the late fetal period and reaches almost the adult level at the time of birth. Studies on in vitro transcription by the nuclear run-off technique showed that the developmental increases in the mRNAs of tryptophan oxygenase and albumin are caused by an increase in the rates of transcription of their genes. Treatment of rats with cortisol significantly increased the amount of tryptophan oxygenase mRNA in the liver from soon after birth. This treatment did not increase mRNA of albumin. It is suggested from these findings that the gene of tryptophan oxygenase is switched on as early as the first day after birth in the few differentiated hepatocytes present in the liver and that the number of these differentiated cells gradually increases during early postnatal development. Although injected glucocorticoid stimulated transcription of the gene of tryptophan oxygenase precociously during this period, presumably in vivo the activity of tryptophan oxygenase normally increases about 2 weeks after birth, because this is when the plasma concentrations of glucocorticoid and glucagon increase sufficiently to be effective.