Testosterone injection caused a 2,000% increase in renal ornithine decarboxylase activity in intact male mice. A single injection of testosterone produced the same effect as repeated injections. The response was dose-dependent and could be blocked by actinomycin, diaminopropane, and cadaverine. Cycloheximide and putrescine had no inhibitory effect. Renal ODC response to arginine vasopressin was altered after castration; however, urine specific gravity and serum osmolality were unaffected by changes in renal ornithine decarboxylase activity.