Summary Dieldrin was detected in the milk of four cows when fed liay made from forage that had been sprayed with 3.5 and 7.0oz. of dieldrin per acre. The average daily dieldrin intakes were 0.10 and 0.75mg. per kilogram of body weight when the cows were fed the hays sprayed with 3.5 and 7.0oz. of dieldrin per acre, respectively. Dieldrin in soybean oil solution was administered by capsule to cows in amounts ranging from 0.11 to 2.32mg. of dieldrin per kilogram of body weight. The concentration of dieldrin in the fat-corrected milk ranged from 1.7 to 13.1 p.p.m. Dieldrin excretion in the milk of animals fed from 100 to 1,000mg. of dieldrin daily continued for more than 47 days after cessation of feeding.