Silage produced from com treated in the field with dimethoate contained the insecticide in amounts that were quite stable in storage. About 50% of the insecticide remaining on the plants at the time of ensiling (1 day after treatment) was still present after storage for 80 days. When corn silage containing 1-7 ppm of dimethoate was fed to dairy cows and beef steers for 28-42 days, daily ingestion of insecticide was estimated to range from 0.06 to 0.36 mg/kg of animal weight. Milk from these cows exmined periodically by analytical procedures sensitive to about 0.03 ppm contained no detectable residues of dimethoate. These amounts of the insecticide produced no statistically significant effect on the blood cholinesterase activity of the dairy cows and beef steers, and there was not any impairment of animal health or performance that could be associated with ingestion of these residues. Capsule feeding of 0.50 mg/kg dimethoate and 0.05 mg/kg of its oxygen analog for 14 days did not produce residues in milk. When these amounts were doubled and fed an additional 14 days, residues of the oxygen analog ranged from 0.004 to 0.125 ppm, but no dimethoate was detected; 3 days later, no oxygen analog was detectable.