Stimuli known to induce tyrosine aminotransferase in H35 cells were tested relative to their ability to induce ornithine decarboxylase, the initial enzyme in the polyamine biosynthetic pathway. Dibutyryl cyclic[c]AMP (0.5 mM), p-chlorophenylthio-cyclic AMP (0.1 mM) and dexamethasone (1 .mu.M) stimulated the activity of ornithine decarboxylase 7 to 8 fold by 5 h of induction. There was a delay of 1 h before any increase in enzyme activity was detectable. Insulin administered alone failed to significantly change ornithine decarboxylase activity. The ability of dibutyryl cAMP to elevate ornithine decarboxylase activity was concentration-dependent, and a dose-response relationship very similar to that for the induction of tyrosine aminotransferase by dibutyryl cAMP was observed in these cells. The ability of various 8-substituted cAMP analogs to increase the activity of ornithine decarboxylase was correlated with their ability to activate purified protein kinase.