Heatstroke is an illness that occurs when body temperature is grossly elevated, causing widespread tissue damage. The extent of tissue damage depends on the level of body temperature elevation and the duration. Despite the fact that the diagnosis of heatstroke is based on sound scientific principles, namely the elevation of serum enzyme levels as indicators of tissue damage, the sensitivity of these parameters of tissue damage in the prodromal period of heatstroke is less well established, especially for sub-lethal stress conditions. Furthermore, it is not known to what extent given elevations in serum enzyme levels reflect the nature of various combinations of hyperthermia and its duration as sustained during the prodromal period. In an attempt to throw some light on the questions posed above anaesthetized rats were exposed to three different sets of thermal conditions. However, the amount of heat gained over and above baseline levels was controlled to a 20% rise irrespective of the experimental conditions. Above this increment animals did not survive thus indicating excessive stress. Plasma enzyme levels were assayed in each group of animals upon termination of stress, six hours post-stress and 24 hours post-stress in order to investigate the patterns of enzyme release as well as the sensitivity of the respective indicators of tissue damage. On the basis of plasma enzyme assays, the tissue damage sustained during these particular experimental conditions was mild to moderate, completely reversible, not indicative of heatstroke but merely of generalized tissue damage. The results suggest that in addition to the established positive relationship between the level and duration of hyperthermia and tissue damage, a third component,namely the rate of rise in body temperature, nay constitute an important factor in the ultirrate pathology. In this regard, i.e. sub-lethal stress, creatine kinase proved to be the most sensitive and, therefore, the most useful parameter of tissue damage. / Thesis (M.Sc.)-University of Durban-Westville, 1984.