Nitrogen oxide-containing compounds displaced the peripheral benzodiazepine ligand [ 3 H]Ro5-4864 from guinea pig membrane preparations. Sodium nitroprusside (SNP) was the most potent (IC 5 0 = 5.61 +/- 1.72 x 10 - 5 M). Moreover, its ability to bind to these receptors showed marked tissue variability (heart > kidney cerebral cortex). When tested on rat atrium, SNP by itself had no effect on basal inotropy or the increase in inotropy induced by (-)-S-BAY K 8644. In contrast, Ro5-4864 potentiated the marked increase in inotropy induced by (-)-S-Bay K 8644, and SNP completely abolished the potentiation of inotropy observed with Ro5-4864. Since peripheral benzodiazepine receptors are associated with calcium mobilization in the heart, these findings may indicate that some of the clinical effects of nitric oxide-generating drugs could be mediated by these receptors.