Rats have been used frequently as a model for determining the pulmonary response to various stressful situations. It has been shown in the literature that rat lungs are remarkably sensitive to factors which result in the development of gross pulmonary injury, such as stress from exposure to hyperbaric oxygen. Similarly, a high degree of pulmonary reactivity was found in rats after administration of various CNS, convulsants and also exposure to mechanical vibration. The present study was confined to a determination of the gross pulmonary response of rats exposed to mechanical CNS traumatization compared to that of 10 other laboratory species. The frequency and severity of pulmonary injury from CNS traumatization, as indicated by lung weight/body weight ratios and gross appearance, was very striking in the traumatized rats, but we found little or no change in cats, dogs, rabbits and guinea pigs, in Rhesus, squirrel, vervet, and cynomolgus monkeys or in baboons and chimpanzees. Previous studies showed that the gross pulmonary injury from this mechanical head injury in rats, as well as from hyperbaric oxygen exposure, vibration, and CNS convulsants, was ameliorated by the prior administration of various sympathetic blocking agents. The results from the present comparative study tend to suggest that rat lungs are hyper-reactive to this stress.