An earlier study reported an increased resistance of chickens to an acute lethal heating episode (43 degrees C. and 45% relative humidity) during aflatoxicosis. This varies from other stresses investigated which interact with aflatoxicosis to make chickens more sensitive to the stress factor. The effects of graded doses of dietary aflatoxin (0, 0.625, 2.5, 5.0, and 10.0 mug./g.) on the body temperature, on body fat, on the serum glucose, and on the effect of varying the severity of the heat stress were measured. Both serum glucose and total body fat were decreased significantly (P less than 0.05) by doses of 2.5 mug./g. and above. Cloacal temperature was decreased slightly but significantly (P less than 0.05) in chickens fed 5 or 10 mug./g. for 12 days or longer. The mean survival time of birds exposed to a heat stress of 40 degrees C. and 45% relative humidity did not vary with the dose of aflatoxin while a milder stress of 37 degrees C. and 45% relative humidity caused chickens fed aflatoxin at 2.5 mug./g. or above to show decreased survival times (P less than 0.05) in comparison to the controls. These data can be rationalized by assuming that the lessened burden of body fat during aflatoxicosis accounts for the increased survival time in a severe (43 degrees C. and 45% relative humidity) heat stress but that other parameters related to physiological stress play a dominant role during a less severe but more prolonged heating episode. It seems likely that the hypoglycemia, hypothermia, and lessened body fat also account for the previously reported increased sensitivity to a lethal cold exposure during aflatoxicosis.