To investigate the role of body core temperature (Tco) on chlordimeform (CDM) toxicity, experiments were conducted over a range of Tcos. Adult rats were anesthetized with sodium pentobarbital and randomly assigned to one of six equal groups. Groups were paired and Tco was maintained in the rats in each of the respective group pairs at one of three levels. Rats in one group at each temperature level were injected intraperitoneally with 60 mg/kg of CDM. Animals in the corresponding temperature-matched groups received volume-matched injections of normal saline vehicle and served as time-paired controls. The electrocardiogram and heart rate (HR) were monitored throughout the experimental procedure. There was a significant decrease in HR in all CDM-treated groups when compared to the control group animals. The magnitude of the observed cardiac effect was attenuated in the T35 group when compared to that of the other treated groups. Similarly, lethality rates (no. deaths/no. total) for the T37, T35, and T33 groups were 2/5, 0/5, and 3/5, respectively; there were no deaths among the control group animals. From these and previous data from this laboratory, the authors conclude there may be a beneficial effect of hypothermia in rats exposed to toxic agents which is maximal around 35 C.