Stress can be defined as a state of threatened balance induced by external stressor and appear as the display of somatic and psychic reactions, struggling to regain homeostasis. Among stressful stimuli, heat stress is an environmental factor capable of causing a wide range of physiological alteration chiefly at the level of the hypothalamic-pituitary-adrenocortical (HPA) axis. The objective of the present study was to evaluate the effect of acute heat exposure on the ACTH and cortisol levels as well as structurally and ultrastructurally changes of the adrenal cortical glands in rats. Twenty normal adult male albino rats, weighting 180-200 grams, were divided into two equal groups. Group A represented the control rats and group B acted as a heat stressed rats that were exposed to heat at 38-40oC for sixty minutes. At the end of experiment, rats were anesthetized, blood sample withdrawed for hormonal study and suprarenal glands were dissected out and prepared for microscopical and ultrastructural examinations. A significant increase in ACTH and cortisol levels were reported in heat stressed group when compared with control group. Light microscopic examination of suprarenal cortical layers of heat-stressed rats revealed foamy cytoplasm with pyknotic nuclear changes as compared to control rats. In addition, ultrastructure examination of group B showed mitochondrial changes in all zones especially zona reticularis, decreased number of lipid droplets in both zona fasciculate and reticularis, and prominent dilatation of smooth endoplasmic reticulum vesicles when compared with group A. In conclusion, acute heat exposure was a stressful condition affecting the suprarenal glands as evidenced by the altered biochemical hormonal levels along with both structural and ultrastructural changes.