Unidirectional fluxes of [14C]-urate from bath to lumen and from lumen to bath were measured in isolated perfused rabbit proximal tubules. The absorption of urate from the perfusate was small in magnitude and relatively insensitive to alteration by luminal probenecid or D-glucose. By contrast, peritubular probenecid inhibited urate secretion from bath to lumen in all three segments. To estimate the facilitated component of urate secretion for each bath concentration of the anion, the passive component of urate influx (probenecid-resistant influx) was subtracted from the total influx of urate. Urate secretion was strongly dependent on concentration of urate in the bath, the relationship being sigmoidal in many S1 and S2 segments. The S0.5 values of 187,251, and 234 X 10(-6) M obtained from Hill plots of mean data from S1, S2, S3 segments, respectively, indicated that the secretory mechanism had a relatively uniform affinity for urate along the proximal tubule. Vmax values for urate secretion were 568,592, and 55 X 10(-15) mol . min-1 . mm-1 for the same segments. Kinetic analysis indicated that axial heterogeneity of urate secretion is probably due to differences in the relative densities of urate transporters of common affinity along the proximal tubule. We suggest that in intact rabbits with low plasma levels urinary urate is the product of glomerular filtration, passive reabsorption, and minimal proximal secretion. With elevation of plasma urate above endogenous levels, S1 and S2 segments secrete urate at an accelerated rate, possibly reflecting action of a transporter with allosteric features.