The central, peripheral and renal haemodynamic effects after intravenous infusion (1 ml/s) of high doses (4 mg I/kg) of two low osmolar, chemically and physically similar non-ionic contrast media (iohexol and iopentol) were investigated in 16 pigs. Both contrast media induced a significant increase of mean arterial, mean right atrial, mean pulmonary arterial and mean pulmonary arterial occlusion pressure along with an increase in cardiac output. The systemic vascular and pulmonary vascular resistances were decreased. Renal venous blood flow and renal vascular resistance were not significantly changed after infusion of contrast media. There were no differences in haemodynamic effects between the two media studied.