THE number of ova released at ovulation in rats varies within a narrow limit, usually between 10 and 15. Factors which maintain this relative constancy are not known. Several agents capable of inhibiting ovulation are known1,2, but apart from gonadotrophin injections there appears to be no way to increase the normal ovulation rate7. An agent capable of doing this might be useful in improving breeding potential of laboratory or farm animals, and provide a tool for the study of the neuroendocrine basis of superovulation. (Superovulation is defined as an increase in the ovulation rate beyond that found in the controls.) Exposure of mice3, rats4 and hamsters5 to X-rays has been found to increase ovulation rate and/or litter size. As radiomimetic agents6 duplicate several effects of X-rays, they might be expected to induce superovulation in rats. Only Cyclophosphamide (CPA) amongst those we tested had this effect.