Measurement in the rat of renal artery-to-vein transit times permitted evaluation of movement of 14C-labelled urate and of other solutes out of the peritubular inulin spaces in the renal cortex. No interaction between urate and PAH could be demonstrated at this level. Provided the peritubular disappearance may be equated to cellular uptake a mean unidirectional urate flux across peritubular membranes equal to about 1/3 of total renal load may be calculated. This value greatly exceeds the net secretory flux of urate. Such a fact can be explained by the likelihood of significant post-proximal reabsorption of secreted urate, and/or by the conclusion that peritubular urate permeability exceeds that of the luminal side. Assignment of the limiting step in urate secretion to the luminal cell membranes, in contrast to a determining role played by peritubular membranes in PAH transport, can account for the lack of interaction between PAH and urate at the peritubular membrane in rats.