To investigate the sites of renin gene expression and localization of renin in primate ovaries, five cynomolgus (Macaca fascicularis) and one rhesus (Macaca mulatta) monkey were treated with gonadotropins to induce multiple follicle development. One ovary was removed before hCG injection (1200 IU) from three monkeys and one ovary was removed 36 h after hCG administration from three monkeys. In three monkeys, the remaining ovary was removed 3, 5, and 7 days after injection of hCG. To detect and localize renin messenger RNA, 35S-radiolabelled 1.1 kb length complementary DNA and RNA probes of human renin were used for in situ hybridization. To compare the synthesis with the presence and the storage of renin or prorenin, renin antigen was assessed by immunohistochemistry in the same tissues using a polyclonal antibody against human renin (R15). Renin mRNA was detected by in situ hybridization only in ovaries collected within 5 days of exposure to hCG. All such ovaries exhibited a positive signal. Renin mRNA was localized to the theca interna and theca lutein cells. Positive cells were observed in a few growing antral follicles, in occasional mature preovulatory follicles, in corpus luteum, and most strikingly in atretic follicles. No signal was detected in primordial, primary, or in small antral follicles of ovaries exposed to hCG. In contrast with the in situ hybridization data, no signal was detected by immunohistochemistry using antirenin antibodies which exhibited a positive signal in monkey kidney. These results indicate that hCG turns on renin gene expression. Renin is synthesized without significant intracellular storage in monkey ovarian theca interna cells and in corpus luteum. The absence of storage of renin is consistent with the high concentrations of prorenin found in ovarian follicular fluid of hCG stimulated primates and with our knowledge of cellular renin processing which indicate that prorenin is secreted constitutively as it is synthesized.