The experiments were performed on rats of three inbred strains (Wistar, August, and Spreg-Dowley). Arterial pressure was measured during acute emotional stress. The reactivity of arteries to vasopressor stimuli, the corticosteroid function, and the humoral pressor activity of plasma were also studied to investigate some physiologic mechanisms of genetically determined differences in the level of arterial pressure during stress. It was established that the rat strains which were examined did not differ in the level of arterial pressure at rest, but showed marked differences in pressure under conditions of stress: the greatest rise in pressure was noted among August rats, the least among Wistar rats, while animals of the Spreg-Dowley strain occupied an intermediate position. It is concluded that inter-strain differences in the pressor reaction reflect genetical peculiarities in the system of nonspecific stress reactivity of animals.