Although cyclophosphamide (CPA) is an ovarian toxicant, the responsible metabolite(s) have not been identified. The purpose of these experiments was to determine if phosphoramide mustard or acrolein were the proximate toxicants produced by metabolic activation of CPA. To do this analogs of CPA known to generate either phosphoramide mustard or acrolein in vivo were assessed for their ability to produce ovarian toxicity as measured by differential follicle destruction, ovarian volume loss, and uterine weight loss and compared to the effects produced by CPA. Phosphoramide mustard cyclohexylamine salt (PMC) and trans-4-phenylcyclophosphamide (T4P), both of which generate phosphoramide mustard, and didechlorocyclophosphamide (DCPA) and allyl alcohol (AA) which generate acrolein were administered ip to female C57BL/6N mice, 10-12 weeks old, at doses equimolar to 0, 25, 75, 200, or 500 mg/kg of CPA. Three days later the animals were killed, their uterine weights measured and their ovaries removed, fixed, and serially sectioned. Only PMC and T4P produced ovarian toxicity. On an equimolar basis these compounds were over twice as potent as CPA. Both caused a significant reduction in uterine weight (to 50% of controls) at doses of 200 (PMC) and 150 mg/kg (T4P). PMC and T4P also caused a 50% reduction in ovarian volume at doses above 75 mg/kg. Primordial follicles were most sensitive; ED50s were 76.9, 25.3, and 19.3 mg/kg (0.276, 0.091, and 0.069 mmol/kg) for CPA, PMC, and T4P, respectively. Growing follicle numbers were also reduced by T4P and PMC, an effect not seen with CPA treatment. Finally, antral follicles were significantly reduced by all doses of PMC, and with T4P at doses greater than 75 mg/kg. The highest doses of PMC, T4P, and CPA all caused a reduction in antral follicle numbers to less than one percent of controls. Didechlorocyclophosphamide (DCPA) and allyl alcohol (AA), compounds that generate acrolein but not phosphoramide mustard in vivo, had no effect on any of the parameters measured even when injected directly into the ovary. This suggests that phosphoramide mustard is responsible for CPA ovarian toxicity. The greater potency of PMC and T4P compared to CPA is likely the result of these compounds bypassing important detoxification steps, therefore, more of the parent compound reaches the ovary as the toxic metabolite.