The effect of fluoroacetamide (FAA) on the rat testis was investigated. The oral and parenteral routes of administration were studied, and both were found to be effective. It has been demonstrated that low doses of FAA (5 to 20 mg FAA per kg of diet, or daily doses of 5 to 50 µg administered parenterally) produce specific effects on late steps of spermatid development. Step 13 of spermiogenesis was shown to be the most sensitive. High doses of the drug (50 mg per kg of diet or 250 µg sc daily) with time also affect less mature germinal cells, so that after several weeks of treatment only Sertoli cells and spermatogonia remain in the seminiferous tubules.