The uptake of L(+)lactate into rat renal cortical brush border (BBV) and basolateral (BLV) membrane vesicles, isolated through differential centrifugation and free flow electrophoresis, were studied using a rapid filtration technique. In contrast to the lactate transport into the BBV, that into the BLV: 1) was found to proceed only towards equilibrium, 2) showed Na+ -independent coupling of the influx of L(+)lactate and the efflux of L(+) but not to the efflux of D(-)lactate, 3) was not inhibited by D(-)lactate, 2-thiolactate or 3-phenyl-lactate, but 4) was inhibited by 3-thiolactate and alpha-hydroxybutyrate and 5) was accelerated by changes in inwardly directed ionic gradients or by increases in cation conductance both of which led to increased intravesicular positivity. The latter changes had the opposite effect on the uptake of L(+)lactate by BBV. Thus, while the L(+)lactate transport system present in BBV showed the characteristics of Na-dependent electrogenic cotransport system, that in the BLV was consistent with a carrier mediated Na-dependent, facilitated diffusion system.