Whole body gamma irradiation in the rat produced significant whole blood cholinesterase depression on the 10th day at a dosage level of 75 r. The levels tested when plotted and extrapolated indicated threshold changes in cholinesterase activity would be in the vicinity of 20 to 30 r. AET alone, while producing some mild cholinesterase depression, failed to protect whole blood cholinesterase activity from the effects of gamma irradiation at the levels of agent and irradiation tested.