The rate of unidirectional [14C]urea efflux from human red cells was determined in the self-exchange and net efflux modes with the continuous flow tube method. Self-exchange flux was saturable and followed simple Michaelis-Menten kinetics. At 38 degrees C the maximal self-exchange flux was 1.3 X 10(-7) mol cm-2 s-1, and the urea concentration for half-maximal flux, K1/2, was 396 mM. At 25 degrees C the maximal self-exchange flux decreased to 8.2 X 10(-8) mol cm-2 s-1, and K1/2 to 334 mM. The concentration-dependent urea permeability coefficient was 3 X 10(-4) cm s-1 at 1 mM and 8 X 10(-5) cm s-1 at 800 mM (25 degrees C). The latter value is consonant with previous volumetric determinations of urea permeability. Urea transport was inhibited competitively by thiourea; the half-inhibition constant, Ki, was 17 mM at 38 degrees C and 13 mM at 25 degrees C. Treatment with 1 mM p-chloromercuribenzosulfonate inhibited urea permeability by 92%. Phloretin reduced urea permeability further (greater than 97%) to a "ground" permeability of approximately 10(-6) cm s-1 (25 degrees C). This residual permeability is probably due to urea permeating the hydrophobic core of the membrane by simple diffusion. The apparent activation energy, EA, of urea transport after maximal inhibition was 59 kJ mol-1, whereas in control cells EA was 34 kJ mol-1 at 1 M and 12 kJ mol-1 at 1 mM urea. In net efflux experiments with no extracellular urea, the permeability coefficient remained constantly high, independent of a variation of intracellular urea between 1 and 500 mM, which indicates that the urea transport system is asymmetric. It is concluded that urea permeability above the ground permeability is due to facilitate diffusion and not to diffusion through nonspecific leak pathways as suggested previously.