Four experiments examined the effects of stressor predictability on a variety of stress responses, such as stomach ulceration, plasma corticosterone concentration and body weight changes. Rats that received electric shocks unpredictably showed greater somatic stress reactions and more stress-induced pathology than animals that received the same shocks but could predict their occurrence by a signal. Subjects in the Unpredictable and Predictable shock conditions received shock simultaneously through fixed body electrodes wired in series, thus insuring that shock was always of exactly the same intensity and duration for the 2 groups. The results point up the importance of psychologic variables in affecting stress by showing that consequences of the same physical stressor can be markedly altered by psychologic factors such as predictability.