Oral toxicity studies using 13 dairy calves, 1 to 2 weeks old, showed that heptachlor epoxide was approximately 10 times as toxic as heptachlor. Both heptachlor and heptachlor epoxide accumulated to produce poisoning when given in multiple doses. A dosage level of 0.2 mg./kg. of heptachlor epoxide for 100 consecutive days (totaling 20 mg./kg.) produced no harmful effects in a dairy calf. The symptoms and lesions of heptachlor and heptachlor epoxide poisoning were identical to those exhibited by other cyclodiene chlorinated hydrocarbon insecticides. Residues of heptachlor epoxide were found in the fat of all the calves treated with heptachlor or its epoxide. However, chemical analyses for heptachlor epoxide in fat were not a means of diagnosing heptachlor or heptachlor epoxide poisoning.