We studied the effect of Cu,Zn-superoxide dismutase (SOD) and Mn-SOD, which are specific scavenging enzymes of the superoxide anion radical, on ovulation and examined the localization of SOD in rat ovaries. The results were as follows. 1) The number of ova in the Cu,Zn-SOD (8 mg x 4) administrated group (27.8 +/- 5.4 : p less than 0.01) was significantly reduced compared to the control group (49.0 +/- 3.3). Similarly, the number of ova in the Mn-SOD (2 mg x 2) administrated group (16.9 +/- 7.6 : p less than 0.01) was significantly reduced compared to the control (52.9 +/- 6.3). 2) In the rat ovary, Cu,Zn-SOD examined by the immunohistological method was found to be localized in the granulosa cells of mature Graafian follicles, growing follicles, primordial follicles and epithelial cells of the fallopian tubes. Mn-SOD was localized in the external theca cells of mature Graafian follicles and the corpus luteum. The activity of SOD estimated by the modified nitroblue tetrazolium method was consistent with the immunohistological localization of both SODs. We considered that oxygen radicals and SODs play an important role in rat ovulation, and Cu,ZN-SOD and Mn-SOD have a different localization and action in the rat ovary.