Antagonist effect of losartan, a nonpeptide antagonist of angiotensin II, on angiotensin II induced contractile response was studied in five isolated smooth muscle assays. In the rabbit aorta and guinea-pig stomach assays, losartan competed with angiotensin II for the angiotensin receptors in an apparently simple manner, that is compliance with the basic criteria of Schild analysis for simple competition. Noncompliance, however, was observed in the guinea-pig ileum, rat ileum and guinea-pig trachea assays where losartan induced nonparallel rightward shifts of angiotensin II E/log[A] curves and the Schild plots were found to have slopes greater than unity. The observed deviations from simple competitive antagonism appeared to be different from those reported earlier, and a possible explanation involving tissue-dependent noncompetitive factor(s) is discussed.