Sheep were dosed orally with diazinon at 450 and 600 mg/kg: residues of dia. zinon were determined in the blood, brain, liver, kidney cortex and medulla, muscle, fat and rumen contents at intervals after dosing, and cholinesterase activities in the erythrocytes and the brain were determined at the same intervals. The relations between residues, cholinesterase activity and clinical symptoms are discussed from the points of view of their diagnostic value and the possibility of undesirable residues occurring in poisoned animals. Rough estimates of the residues of two metabolites, hydroxydiazinon and dehydrodiazinon, in one sheep are also discussed. The oxygen analogue of diazinon was not detected although it was specifically sought.