Differences in the secretion of pregnane compounds by ovaries with cystic follicles of rats exposed to constant light (light-induced oestrous rats) and by ovaries with normal follicles of early pro-oestrous rats were studied. Some rats were injected iv with 2 micrograms of LH to stimulate the secretion of steroids 30 min before their blood was sampled. The injection greatly increased the secretion of progesterone by both kinds of ovaries. The secretion of 5 alpha-pregnane-3,20-dione and 3 alpha-hydroxy-5 alpha-pregnan-20-one also increased in normal rats, but not in the polycystic ovaries of light-induced oestrous rats, which suggested that the 5 alpha-reductase activity was low. The pre-ovulatory LH surge is absent in light-induced oestrous rats, so a classic approach was taken to circumvent the probable deficit in the cyclic release of LH; we gave multiple injections of 10 micrograms of LH. Five such injections were given at intervals of 4 days, and ovarian venous blood was collected 4 days after the last injection. Cystic follicles in the ovaries of rats disappeared when the injections of LH were given every 4 days. The production of 5 alpha-pregnane-3,20-dione and 3 alpha-hydroxy-5 alpha-pregnan-20-one from the ovaries of such rats was significantly higher (P less than 0.01) than in controls given multiple injections of saline. These results suggest that the low 5 alpha-reductase activity in polycystic ovaries of light-induced oestrous rats may be due to the absence of an LH surge from the pituitary gland.