Tyrosine aminotransferase (TAT) enzymatic activity was undetectable in fetal rat liver until 1 day before birth (21 days). In utero injection of (Bu)2cAMP induced both catalytic and mRNA activity. By contrast, in utero injection of hydrocortisone acetate did not elicit the appearance of TAT enzyme or its functional mRNA. Injection of both the steroid hormone and the cyclic nucleotide elicited the appearance of nearly adult levels of the enzyme and its mRNA. By 24 h after injection of (Bu)2cAMP alone or in combination with hydrocortisone acetate, enzymatic activity had returned to basal levels. Functional TAT mRNA levels, however, remained elevated. The role of insulin as a potential repressor of TAT activity in utero was examined. Reducing circulating fetal insulin levels by injection of streptozotocin was not sufficient to induce TAT enzyme activity. In vitro, either (Bu)2cAMP or hydrocortisone acetate alone induced TAT enzymatic activity in liver explants from fetuses as early as the 16th day of gestation. Explants from fetuses in the 20th day of gestation were able to maintain induced levels of TAT enzymatic activity 48 h after removal from medium containing hydrocortisone.