To investigate the significance of changes in ovarian inhibin in gonadal dysfunction with persistent estrus, female rats with persistent estrus were studied as animal model for polycystic ovaries (PCO). Newborn female Wistar rats treated with testosterone propionate s. c. (TP) and female rats raised under continuous light (LL) started at 9 weeks of age entered into continuous estrus 5 and 2 or 3 weeks after the treatment, respectively. Ovaries in both groups had many polycystic follicles Normal adult female rats with 4 day cycles were served as control. Ovarian inhibin content was measured by FSH suppressing activity of charcoal-treated ovarian homogenates using a rat anterior pituitary cell culture system. Blood samples were obtained at 11:00 a. m. and plasma levels of LH, FSH, PRL, estradiol (E2), progesterone (P) and testosterone were measured by RIA. Comparisons were made statistically between each experimental group and normal proestrus (PE). Both LH and FSH were similar to normal PE in TP LL, suggesting a lack of LH and FSH surges probably through hypothalamic dysfunction in TP and LL. Plasma PRL levels were variably elevated in both groups. Ovarian inhibin contents were comparably high to PE in both groups with parallel increases in plasma E2 and P. Plasma testosterone was elevated only in TP. In conclusion, female rats with persistent estrus induced by TP or LL exhibited polycystic ovarian changes similar to PCO. Increased inhibin content in TP and LL may reflect persistent follicular activity.