The effects of an intracoronary administration of iomeprol, a new nonionic tri-iodinated water-soluble contrast medium, on coronary circulation were compared to those of iopamidol and those of nitroglycerin in 6 chronically instrumented conscious dogs. A pair of 10 MHz piezoelectric crystals and an electromagnetic flow probe were placed on the left circumflex coronary artery (LCCA) to measure the epicardial coronary diameter (CD) and coronary blood flow (CBF). Polyethylene tubing for drug administration was inserted into the LCCA proximal to the sonomicrometers. Iomeprol at the dose of 1 ml and 3 ml/min for 1 min significantly increased CD by 0.6 +/- 0.1% and 1.4 +/- 0.3%, respectively and CBF by 44.5 +/- 9% and 70 +/- 10%, respectively. Iopamidol at the same rates also significantly increased CD by 0.8 +/- 0.1% and 1.5 +/- 0.3% and CBF by 50 +/- 11% and 82 +/- 14%, respectively. There was no statistically significant difference between iomeprol- and iopamidol-induced increases in CD and CBF. However, the duration of the increase in CD was significantly shorter (p < 0.05) after iomeprol than after iopamidol. Nitroglycerin (10 micrograms/kg) significantly increased CD by 4.5 +/- 1% and CBF by 105 +/- 10%. The increases in CD and CBF in response to iopamidol and iomeprol were significantly smaller (p < 0.01) than to nitroglycerin. We conclude that vasodilating effects of iomeprol and iopamidol on the large epicardial coronary artery and coronary blood flow are comparable in conscious dogs and significantly lower than after nitroglycerin in the doses used by us.