Recently we have found [l] that injection of N6, 02’-dibutyryl cyclic adenosine 3’,5’-monophosphate (DBcAMP) into adrenalectomized rats initially stimulated hepatic phosphoenolpyruvate carboxykinase (PEP-CK, EC 4.1.1.32) and tyrosine aminotransferase (TAT, EC 2.6.1.5) to the same degree as in intact rats. However, despite of repeated DBcAMP administration the raise in the activity of both enzymes ceased after 2 hr, but could be restored when hydrocortisone had been given at zero time. These results suggested that the maintenance of CAMP-mediated induction of PEP-CK and TAT is dependent on the simultaneous presence of glucocorticoids. Because of the obvious complications associated with whole animal experiments the hypothesis was now re-examined in the isolated perfused rat liver. It could be demonstrated that the interaction between CAMP and glucocorticoids, in fact, occurred as the result of direct participation of the administered agents at the tissue level. Moreover, our in vitro findings support the hypothesis derived from previous in viva studies [ 11, that CAMP elevates PEP-CK and TAT by a mechanism which, in principle, works independently from glucocorticoids while glucocorticoids stimulate a different process that is required for the maintenance of CAMP-mediated enzyme induction. 2. Materials and methods