In the guinea-pig isolated ileum amrinone (3 microM to 0.3 M) induced transient, dose-dependent contractions which were followed by pronounced relaxation only at the highest amrinone concentrations tested (30 mM to 0.3 M). The contractile effect was potentiated by eserine and abolished by scopolamine. Verapamil, at concentrations (0.1-1 microM) that did not affect basal tension of the preparations or the contractile response to exogenous acetylcholine, selectively reduced amrinone-evoked contraction, while the relaxing component was abolished by verapamil concentrations (5-10 microM) that per se reduced muscle tension as well as acetylcholine-induced contraction. Similarly, when external Ca2+ concentration was reduced from 2.5 to 1.25 and 0.5 mM, the contractile component of amrinone effect was markedly reduced, while relaxation was not significantly altered. Any further decrease of Ca2+ content in the superfusion medium (0.25 to 0 mM) also reduced the relaxing effect of amrinone and the response of the ileum to exogenous acetylcholine. The present results indicate that in guinea-pig ileum amrinone-induced contractions are sustained by acetylcholine release as well as by Ca2+ entry into smooth muscle cells through verapamil-sensitive channels, while other Ca2+ pools may play a role in the relaxing effect of the drug.