Erythrocyte uridine transport has been studied in eight normal individuals and eight patients on haemodialysis for chronic renal failure. The initial rate of zero-trans uridine influx at 37 degrees C has been measured as a function of extracellular uridine concentration using [14C]-labelled uridine. The results are consistent with Michaelis-Menten kinetics. In normal humans the mean Vmax for uridine influx was 32.8 +/- 6.4 mumol (1 cells)-1 s-1 (mean +/- S.D.) and the mean Km was 190 +/- 12.3 microM. The measurements made in renal failure patients were not significantly different (mean Vmax 30.1 +/- 7.1 mumol (1 cells)-1 s-1, mean Km, 185 +/- 13.2 microM). These results are discussed with reference to the reported data on uridine transport in human erythrocytes at temperatures between 4 and 35 degrees C; it is suggested that zero-trans uridine influx shows a decrease in temperature dependence above 25 degrees C. The Vmax for zero-trans uridine influx at 37 degrees C in normal erythrocytes represents a turnover number for the nucleoside transporter of 180 uridine molecules per second.