Various hormones have been shown to enhance the incorporation of labeled amino acids into protein of their target organs. The precise mechanism or the exact site at which these hormones act to stimulate protein synthesis is not known for any of them. The hormone which has been most carefully studied in this respect is insulin. Thus, it has been shown that this hormone stimulates the incorporation of labeled amino acids into protein of isolated rat diaphragm (1,2,3,4). When it was found that insulin also stimulates the accumulation of normal amino acids and of the non-utilizable amino acid α-aminoisobutyric acid (AIB) by cells of the isolated, “intact” diaphragm, the possibility was considered that the increased protein synthesis might be caused by increased amino acid concentration (5,6,7). However, other observations have failed to support this possibility and have led instead to the conclusion that insulin primarily stimulates the rate of intracellular protein syntheis and that the increased uptake of amino acids is secondarily induced by the effect of insulin in stimulating the rate of protein synthesis (8). It has recently been reported that follicle stimulating hormone (FSH) under certain experimental conditions enhances the incorporation of labeled amino acids into the protein of isolated rat ovaries (9,10), and, further, that under similar conditions this hormone also stimulates the accumulation of utilizable and non-utilizable amino acids (10,11). In relation to the above mentioned theories concerning the mechanism of action of insulin in stimulating protein synthesis, it seems important to discover whether FSH acts primarily on the rate of protein synthesis in the ovarian cells or on the transport of amino acids. In the experiments which will be presented in this paper it was found that FSH can stimulate the accumulation of utilizable and non-utilizable amino acids by the isolated rat ovary even when protein synthesis has been inhibited by puromycin.