The steroid hormone profile of Scotophilus heathi, and Indian tropical vespertilionid bat, was studied, with special reference to the period of delayed ovulation. The results show unusually high concentrations of circulating androstenedione and testosterone during the first half of the period of delayed ovulation, whereas the concentrations of progesterone and oestradiol were relatively low during this period. Androstenedione, testosterone and oestradiol concentrations were high during the period of ovarian recrudescence (October-November). The oestradiol concentration increased, whereas androstenedione and testosterone declined significantly, before ovulation. Studies in vitro showed that the ovary secrets more androstenedione and testosterone than does the adrenal. A histological study showed that a major part of the ovary of S. heathi was occupied by hypertrophied interstitial cells. An immunocytochemical study showed ovarian thecal interstitial cells as the major site of high androgen synthesis in the ovary. During the period of delayed ovulation the bat ovaries thus resemble the ovaries from women with polycystic ovaries in which abnormally high androgen secretion from hypertrophied interstitial cells prevent ovulation.