Polyadenylation of mRNA is a post-transcriptional phenomenon and is catalysed by both nuclear and cytoplasmic poly(A) polymerases. In rat testis the specific activity of cytoplasmic poly(A) polymerase increased with age reaching a peak value in 80 days. After this time the activity was maintained at a slightly lower level for the next 280 days. Follicle stimulating hormone, luteinizing hormone, human chorionic gonadotropin, pregnant mare serum gonadotropin and a gonadotropin releasing hormone analogue significantly increased the enzyme activity in immature rat testis. These results show that testicular poly(A) polymerase was probably under the influence of gonadotropic hormones.