1. The action of angiotensin on the circular and longitudinal muscle of the human colon has been studied in vitro. The ED50 value (g/ml) of circular muscle was 8.7 x 10(-8) and of longitudinal muscle 3.1 x 10(-8), while the corresponding figures for acetylcholine were 1.6 x 10(-6) and 1.1 x 10(-6), respectively. Angiotensin caused a sustained contraction associated with prolonged tachyphylaxis.2. As the action of angiotensin was not antagonized by atropine, hexa-methonium or procaine, angiotensin appeared to act directly on the smooth muscle of the human colon.3. Lidoflazine is not a specific antagonist of angiotensin.