Materials and methodsMale Wistar rats Rattus norvegicus Berkenhaut 1769, weighing 200±20·g, were acclimated to 22±1°C, maintained under a 12·h:12·h light:dark regime, and fed a commercial rat food and water ad libitum. The rats were divided into two groups, each consisting of five animals. The first group were intact controls. The rats from the second group were exposed to an ambient temperature of 38°C for 60·min before sacrifice.Animals were weighed and decapitated. The body mass of control rats was 203±9.79·g (mean ± S.E.M.), and 190.2±7.90·g for heat stressed rats. Animals were decapitated by guillotine (Harward-Apparatus, Holliston, MA, USA). The left adrenal gland was quickly excised, freed of fat tissue (4°C) and weighed. Blood was collected from the trunk, divided into two sets of tubes, and EDTA added to obtain plasma. Serum and The morphological and stereological structure of rat adrenal gland was analysed by light microscopy after an acute (60·min) exposure to high ambient temperature (38°C). A significant increase in plasma corticotrophin (ACTH) and serum corticosterone (CORT) concentrations was observed, confirming that acute heat exposure has a strong stressful effect. Under these conditions the adrenal gland mass and volume were decreased, probably as the consequence of adrenal cortex reduction, especially that of the zona fasciculata (ZF). Histological examination revealed that many ZF cells were deprived of lipid droplets. Fibrosis was observed in all parts of the adrenal gland, both cortex and medulla, of heat stressed animals. Mitotic figures were absent in cortical cells after heat exposure, but there were no differences in ZF and zona reticularis (ZR) small blood vessels compared to nonstressed controls.