Urate transport was studied in isolated perfused S2 segments of the superficial proximal tubule of the rabbit. When urate was present in identical concentrations of 290 microM in the perfusing and bathing solutions, there was a net secretory flux of urate of 775.0 +/- 152.8 fmol.min-1.mm-1. When urate was present in varying concentrations in the bathing solution only, the bath-to-lumen flux of urate increased as the concentration of urate in the bathing solution was increased from 60 to 595 microM, but tended toward a plateau at higher concentrations. After correction of the net secretory flux for the contribution of passive permeation, an apparent Km of 238 microM and Vmax of 950 fmol.min-1.mm-1 for the secretory flux of urate was calculated. When urate was present only in the luminal perfusion solution, the lumen-to-bath flux of urate was significantly less than the bath-to-lumen flux at a similar bath concentration of urate. The lumen-to-bath flux of urate was not significantly affected by cooling the bath to 25 degrees C. These studies provide evidence for both passive and facilitated mechanisms for urate secretion in the rabbit S2 segment. The absorptive flux for urate, however, appears to be primarily a passive mechanism.