A 6 h immobilization stress was found to be lethal for rats weighing 125 g in average. Stress of 4 h was sublethal for the majority of these rats, however, simultaneous administration of adrenaline turned the sublethal effect to lethal. Death occurred in the fifth or sixth hour measured from the beginning of the stress, independently of the fact whether the stress was going on or not. Severe morphological changes in the heart muscle, lungs and kidneys were found.