Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and Nh,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt,cAMP), whereas less mature hepatocytes ( Biochem. 199,. Therefore, we consider >El5 hepatocytes, and not