Blood flow in normal skin and skin treated with ultraviolet light (UVL) has been determined by measuring the clearance of epicutaneously applied xenon 133 gas. Mean blood flow in UVL-irradiated skin was 25.7 ml/100 gm/min, which is about twice that found in normal skin (12.2 ml/100 gm/min). Orally administered indomethacin reduced by one third the increase in blood flow produced by UVL irradiation.