The role of RNA synthesis was investigated in the early action of FSH on testes of immature rats. FSH was demonstrated to accelerate the testicular incorporation of 3Hcytidine into rapidly labeled nuclear RNA within 15 min of a single injection. Stimulation of the rate of 3H-cytidine incorporation was accompanied by a change in the base ratio of the nuclear RNA (i.e., more A + U). Moreover FSH increased the extent of the testicular chromatin available for transcription. Chromatin content of acidic (nonhistone) protein appeared to vary inversely with template efficiency in response to FSH whereas no changes were noted in histone content. The stimulation of testicular RNA synthesis was specific for FSH and was abolished by actinomycin D. Analysis of the rapidly labeled nuclear RNA by polyacrylamide gel electrophoresis demonstrated this RNA to be of high molecular weight (greater than 28S) and relatively undegraded. However this technique failed to reveal major qualitative differences in RNA purified from nuclei isolated from control or FSH-treated rats. Similarly, no large qualitative differences could be demonstrated by gel electrophoresis in the proteins synthesized in vitro 1 hr following FSH administration or in the peptides synthesized in vitro by testicular polyribosomes isolated from unstimulated or 1-hr FSHtreated rats. It is suggested that these results would be compatible with a testis-specific stimulation of nuclear transcription as an early event in the action of FSH. (Endocrinology89: 981, 1971)