Suppression of delta 5-3 beta-hydroxysteroid dehydrogenase (delta 5-3 beta-OHD) and glucose-6-phosphate dehydrogenase (G-6-PD) activities were observed in the rat ovarian tissues following treatment with Mitomycin C (MC), an antibiotic which depresses DNA synthesis. The same treatment also resulted in accumulation of cholesterol and ascorbic acid in the ovaries and a decrease of uterine weight. The atretic changes in the treated ovaries were judged from the activity of the lysosomal enzyme leucine amino-peptidase (LAP). The results suggest a diminution in ovarian steroid biogenesis following an alteration of DNA synthesis.