Purine analogues were used to probe the mechanism by which nerve growth factor (NGF) and other agents regulate cellular ornithine decarboxylase (ODC) activity. Exposure of cultured rat pheochromocytoma PC12 cells to NGF causes a 10-50-fold induction of ODC activity within 4-6 h. We recently found that purine analogues block this induction as well as other, but not all, actions of NGF and have provided evidence that the inhibitory actions of the analogues may be due in part to the suppression of an NGF-activated protein kinase activity (Volonté, C., Rukenstein, A., Loeb, D. M., and Greene, L. A. (1989) J. Cell Biol. 109, 2395-2403). The present results show that the purine analogues also suppress the induction of ODC mRNA. One of the analogues used was 6-thioguanine (6-TG). Although 6-TG was effective when applied simultaneously with NGF, if NGF was administered for as little as 1-3 min before 6-TG, ODC induction was unimpaired. This suggests that 6-TG blocks an early step in the NGF mechanism, and that once this step is triggered, the ODC induction pathway is no longer sensitive to this analogue. In contrast, another purine analogue, 2-aminopurine (2-AP), effectively inhibited ODC induction even if applied only during the last hour of a 5-h exposure to NGF. It is hypothesized that this increased period of sensitivity to 2-AP may be due to its broader range (as compared to 6-TG) as an inhibitor of protein kinase activities. Epidermal growth factor (EGF) and cAMP derivatives also induce ODC activity in PC12 cells, and these effects were suppressed by 6-TG and 2-AP at concentrations similar to those that affect responses to NGF. However, short term (less than 30 min) pretreatment with EGF or a cAMP derivative did not protect induction of ODC activity by these agents from inhibition by 6-TG. This suggests that there are both convergent and divergent elements in the mechanistic pathways used by NGF, cAMP analogues, and EGF to induce ODC.