We examined the effects of acute and long-term exercise on perceptions of personal efficacy in sedentary, middle-aged males and females. Both males and females demonstrated significant increases in efficacy following acute exercise. Females, who had demonstrated initially lower self-perceptions than males, made dramatic increases in efficacy during the exercise program, equaling or surpassing those of males. Exploratory analyses revealed significant relationships between self-efficacy and subsequent physiological responses to exercise.