In this study, the effects of an aqueous-ethanol extract from Crocus sativus on heart rate and contractility were examined. Isolated guinea-pig hearts were perfused through the aorta in a Langendorff mode. Heart rate and contractility were determined in the presence of four concentrations of the extract (0.1, 0.5, 1.0 and 5.0 mg%) and diltiazem (0.1, 1, 10 and 100 microm) in perfused heart with: (1) ordinary Krebs solution (group 1, n = 9), (2) calcium-free Krebs solution (group 2, n = 7). In group 1, three higher concentrations of diltiazem (1, 10 and 100 microm), but only the highest (5.0 mg%) and two higher concentrations (1.0 and 5.0 mg%) of the extract caused significant reduction in heart rate and contractility, respectively (p < 0.05 to p < 0.001). In group 2, the highest (100 microm) and two higher concentrations (10 and 100 microm) of diltiazem (p < 0.05 to p < 0.01), but only the highest concentration of the extract showed significant reductions in the heart rate and contractility (p < 0.05 to p < 0.01). There were significant negative correlations between concentrations of the extract and diltiazem and their effects in both groups (p < 0.01 to p < 0.001). These results suggested a potent inhibitory effect of aqueous-ethanol extract from C. sativus on the calcium channel of guinea-pig heart.