Using continuous-flow luminal microperfusion techniques, the influence of the intraluminal urate concentration on urate absorption was determined in the rat proximal tubule. When the estimated contribution of passive permeation was accounted for, the "active" component of urate absorption demonstrated saturation kinetics. The apparent Km was 0.17 mM and the Vmax 0.31 pmol.min-1.mm-1. These transport constants were similar when derived from either a water-absorbing or steady-state equilibrium perfusion solution. The reflection coefficient was determined in studies employing the techniques of simultaneous capillary and luminal microperfusion. Both perfusion solutions contained p-chloromercuribenzoate to inhibit active urate transport. In the presence or absence of an osmole gradient imposed across the tubule, the reflection coefficient for urate averaged 0.94. These studies provide evidence that urate absorption in the rat proximal tubule is a carrier-mediated process. They also provide independent confirmation of the passive flux coefficient derived in prior studies. Finally, the results suggest that solvent drag would have little effect on urate absorption.