Although tepa, metepa, and apholate are only moderately toxic to rats by the oral route, the dermal toxicity of the first two is relatively high as compared with that of conventional insecticides. In the experiments reported, metepa caused no damage to he intestinal epithelium of rats, except following single doses in the fatal range. The organ primarily affected by repeated small doses of metepa is the testis; the ovaries and bone marrow are damaged only by higher dosage levels. Metepa at an oral dosage of 5 mg/kg/day (about 4% of the acute oral LD50 level daily) produced severe reduction of fertility of male rats within 22 days, sterility within 70 days, and testicular atrophy within 77 days. Half that dosage produced a smaller reduction in fertility and only partial testicular atrophy in some rats within 197 days. Dosages of 1.25 mg/kg/day or less produced no detectable effect on fertility and no histological change in the testis in 197 days. The survival of newborn rats was not affected by any dosage given to their sires.