Contractions were elicited by field stimulation and norepinephrine in canine cutaneous arteries in Krebs-bicarbonate and Krebs-Tris media and the effect of angiotensin II, 1 × 10<sup>–7</sup> m, on these responses was determined. The absolute magnitude of the field stimulation responses was much greater in the bicarbonate buffer. Angiotensin caused potentiation in both media of the responses to field stimulation and norepinephrine. In Tris buffer with 5 mM calcium the responses to field stimulation and norepinephrine were greater than in 2.5 mM calcium, however; the degree of potentiation caused by angiotensin was similar at both calcium concentrations. The angiotensin potentiating effect in vitro appears to involve a postsynaptic action of the polypeptide which is not affected by alteration of calcium concentration or the medium.