It has been suggested that nephrotoxic effects of X-ray contrast media (CM) are exerted via renal medullary hypoperfusion and hypoxia. This study was therefore undertaken to investigate the effects of CM injection on renal medullary microcirculation and oxygen tension. Red blood cell velocity (VRBC) in the renal medullary vessels was measured by a cross-correlation technique and was found to be decreased by mannitol and by all CM tested except ioxaglate. The incidence of red blood cell aggregation and cessation of blood flow in the renal medullary vessels was estimated with use of a score based on visual observations. All tested CM and mannitol induced aggregation in the renal medullary vessels. The ionic CM ioxaglate was found to induce the least aggregation. Modified Clark PO2 microelectrodes with a guard cathode were constructed and adapted for tissue PO2 measurements. PO2 was measured simultaneously in the renal cortex and outer renal medulla. The PO2 in rats with blood pressure (BP) above 80 mm Hg was found to be lower in the medulla than in the cortex. In rats with BP below 80 mm Hg the PO2 was paradoxically higher in the outer medulla than in the cortex, probably due to a decrease in the glomerular filtration rate and hence in sodium uptake from the tubules. After injection of all tested CM, PO2 in the renal cortex remained unchanged, while in the outer medulla it decreased. There was no difference in the decrease in PO2 between the tested CM. The decrease in outer medullary PO2 was prevented by injection of furosemide but not by injection of mannitol. A decrease in VRBC and PO2 in the renal medulla may be an underlying cause of CM-induced acute renal failure.