Recent reports emerging from several laboratories have strengthened the concept of a physiological intrinsic ovarian renin-angiotensin system. Encouraged by these studies carried out mostly so far in human subjects, we decided to investigate if prorenin- and renin-like activities could be demonstrated in the follicular fluid obtained from bovine ovaries. The results obtained in this study show that in a total of 58 follicles examined, significant quantities of both prorenin- and renin-like activities could be demonstrated. The prorenin-like activity measured was invariably 30-40 times greater than the renin-like activity in the follicular fluid samples. There was an inverse relationship between the ratio of estradiol to progesterone concentrations and prorenin-like activity in the follicular fluid. Similarly, a significant negative correlation was seen between the estradiol concentration and prorenin-like activity in follicular fluid. On the other hand, there was no discernible relationship between androstendione concentrations and prorenin-like activity. With respect to progesterone, in large follicles with high (greater than 50 ng/ml) progesterone content, the prorenin-like activity was 3 times as high as that in the low (less than 50 ng/ml) progesterone group. There was, however, no difference in the levels of prorenin-like activity between the high and low progesterone groups in the case of small follicles. Furthermore, analysis of the contents of prorenin- and renin-like activities in extract prepared from granulosa and thecal cells revealed thecal cells of bovine ovarian follicles to be a major source of the enzyme activities. The ratio of prorenin to renin activity in thecal cell extract was close to 1. By analyzing the relationship between various steroid concentrations and prorenin-like activity in follicular fluid, it appears that the atretic follicles are likely to have significantly higher levels of prorenin-like activity in their fluid.