Two new synthetic analogs of Asp1-Ile5-angiotensin II, Sar1-Ile5-Ile8-angiotensin II and (N,N-dimethyl) Gly1-Ile5-Ile8-angiotensin II which are chemically related, have a potent competitive antagonistic action against Asp1-β-amid-Val5-angiotensin II on the rabbit isolated aorta, rat ascending colon, isolated perfused heart of cat and in vivo on the blood pressure and intestinal motility of the chloralose-anesthetized cat. Compared to Sar1-Ile5-Ile8-angiotensin II, (N,N-dimethyl) Gly1-Ile5-Ile8-angiotensin II has equal antagonistic potency on the isolated preparations but a lower potency in vivo. The duration of the antagonistic effect of (N,N-dimethyl) Gly1-Ile5-Ile8-angiotensin II in all preparations investigated has been found to be significantly shorter than that of Sar1-Ile5-Ile8-angiotensin II. It is assumed that replacement of sarcosine with N,N-dimethyl glycine at the NH2-terminal position, enhances the degradation of the analog by aminopeptidase and therefore decreases its half-life.