Nerve growth factor treatment produces a large increase in the activity of ornithine decarboxylase and a moderate decrease in the activity of S-adenosylmethionine decarboxylase in PC12 cells. These changes are reflected weakly, if at all, in the levels of putrescine, spermidine, and spermine in the cells. The rates of polyamine synthesis are increased somewhat more than the overall levels, but still are not comparable in extent to the increase in the ornithine decarboxylase activity. Inhibitors of ornithine decarboxylase and S-adenosylmethionine decarboxylase have their expected effects on the induction of ornithine decarboxylase and on the activities of both enzymes. Neither inhibitor alone, nor a combination of inhibitors, altered the rate or extent of nerve growth factor-induced neurite outgrowth in the cells.