To determine whether vitamin E (di-alpha-tocopherol) supplementation could protect against short-term respiratory responses to O3 exposure its effects were investigated in young healthy adult volunteers. Experimental groups received 800 or 1600 IU vitamin E per day for 9 or more wk, while control groups received placebo. Subjects were then exposed for 2-h periods to 0.5 ppm O3, with secondary stresses of heat and intermittent light exercise. Responses to O3 exposure, evaluated in terms of symptoms, forced expiratory performance, and single-breath nitrogen washout, were not significantly different between vitamin E and placebo groups.