Iloprost is a prostacyclin analogue mainly used in the treatment of pulmonary hypertension. It is also effective for renal dysfunction during renal transplant and coronary artery bypass grafting surgery, but there have been few comparative experimental studies with iloprost. We aimed to compare vasorelaxant properties of iloprost, with those of diltiazem, nitroglycerin and papaverine on rat thoracic aorta following phenylephrine-induced precontraction. Thirty-two young adult female Wistar albino rats were used in the study. After isolation of thoracic aorta distal to arcus, 3 mm aortic rings were placed in an organ bath and phenylephrine (10(-6) M) was added for precontraction. Subsequently, cumulative concentrations of iloprost, nitroglycerin, diltiazem and papaverine were added to aortic rings on separate chambers and dose-response curves were recorded. The mean (+/- SD) maximal relaxation (E-max) was 27.1 +/- 2.9% for iloprost, 111.7 +/- 2.1% for nitroglycerin, 77.4 +/- 2.9% for diltiazem and 147.2 +/- 2.8% for papaverine. The half maximal effective concentration (EC50) values were 2.4 x 10(-11) molar (M) for iloprost, 5.6 x 10(-8) M for nitroglycerin, 7.0 x 10(-7) M for papaverine and 2.1 x 10(-5) M for diltiazem. There was a statistically significant difference in Emax values and EC50 values between four groups (P<0.001 and P<0.01, respectively). Although a vasorelaxant response was observed with iloprost, it was less compared to papaverine, nitroglycerin and diltiazem on the isolated rat aortic rings. Iloprost has also the lowest EC50 value which could be valuable in clinical practice.