The effects of 19 nitrate esters of genins and glycosides on the isolated guinea pig atrium and ileum were studied. The positive inotropic effect of glycoside nitrates correlated with the contracting effect on the ileum. Genin nitrates with weak cardiac activity were found to exert smooth muscle relaxing effects. Compared to genin nitrates, glyceryl trinitrate had a weaker relaxing effect on the ileum. On the coronary strip it had a stronger effect than Δ14-anhydrodigitoxigenin-3β-mononitrate, the most effective smooth muscle relaxing genin nitrate.