The present study was undertaken to clarify the physiological role of the free radical-scavenger system in the control of estrogen production in the human ovary with age. Both superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities in the homogenate of the ovary significantly decreased from the premenopausal period to the menopausal period. The lipid peroxide content, however, showed a small and gradual decrease from the premenopausal to the postmenopausal period. In the premenopausal women, aromatase (Ar) activity in the microsomal fraction of the ovary showed a sharp decline and a significant negative correlation with age. Significant positive correlations were found between SOD and Ar activity, and between GSH-Px and Ar activity. Treatments of homogenate of the ovary with 10, 50 and 100 µ M of H<sub>2</sub>O<sub>2</sub> resulted in a decrease in Ar activity in a dose-dependent manner. Moreover, treatment with 10 and 50 µM of H<sub>2</sub>O<sub>2</sub> produced a more significant decrease in the Ar activity in the premenopausal women than that in the postmenopausal women. Treatments with 0.01, 0.1, 1.0 and 10.0U of GSH-Px before administration of 50 µM of H<sub>2</sub>O<sub>2</sub>, on the other hand, induced a recovery of the Ar activity in a dose-dependent manner. The present study showed the inhibitory effect of peroxide on Ar activity, which was attenuated by GSH-Px. These results seem to suggest that these age-related, relatively free radical predominant environments in the human ovary may have an important role in the decrease of estrogen production through the effects on the Ar activity.