1. Spirally-cut strips from human visceral arteries isolated in vitro were contracted by noradrenaline and this contraction was antagonized by the alpha-receptor blocking agent, thymoxamine.2. The cumulative log dose-response curves to noradrenaline in the presence of thymoxamine were moved to the right of and parallel to the curve for noradrenaline alone. The blockade was surmountable and reversible.3. The quantitative criteria for simple competitive antagonism were satisfied. When log (x - 1) was plotted against pA(x) the points fitted a straight line with a slope (-n) of 0.91. The pA(2) for thymoxamine was 7.54 and the mean value for log K(2) was 7.82. These values are in approximate agreement with those previously reported for animal isolated arterial strips.