Ornithine decarboxylase, the initial enzyme in the polyamine biosynthetic pathway, is induced in target tissues in response to a variety of trophic agents including polypeptide and amine trophic hormones, cyclic AMP analogs, drugs, and trophic steroid hormones. The induction of ornithine decarboxylase in these systems is regulated at a transcriptional level and is proportional to the extent of stimulation. Because of its rapid half-life (10-20 min), a general maximum of induction is detectable within 4-5 h of stimulation, and its induction pattern can serve as a rapid, specific index of increased RNA and protein synthesis. Implications for its usefulness to pharmacologists, endocrinologists, physiologists, and biochemists are summarized.