An experiment was designed to determine the presence or absence of parathion in the milk of dairy cows fed parathion in capsules. Ten dairy cows in heavy lactation, representing four of the major breeds, were allotted into two groups and fed a commercially available wettable powder formulation of parathion at levels of five ppm. and one ppm., based upon an estimated roughage dry matter intake of 2.25 lb. per 100 lb. of body weight daily, continuously for 81 days. These feeding levels represent an actuaul parathion intake of 0.112 mg. of parathion per kilogram of body weight for the cows receiving five ppm. of parathion, and 0.022 mg. of parathion per kilogram of body weight for the cows receiving one ppm. of parathion based upon the estimated roughage dry matter intake. At the conclusion of this experiment, two of these cows were fed increasing amounts of parathion up to 40 ppm. of the estimated roughage dry matter intake. In neither experiment was any parathion found in the milk of the experimental cows by the use of both a chemical method of analysis for parathion and biological assay using adult houseflies. No objectionable flavor was noted in the milk and no harmful effects to the health or reproductive ability of the cows were observed.