Rat proximal convoluted tubules were microperfused in vivo to examine the effect of flow rate on bicarbonate absorption. When tubules were perfused with 25 mM bicarbonate, increases in perfusion rate from 15 to 33 to 49 nl/min caused bicarbonate absorption to increase from 105 +/- 4 to 176 +/- 8 to 209 +/- 7 pmol X mm-1 X min-1, respectively. Only 15% of this stimulation could be attributed to a flow-induced increase in the measured axial luminal bicarbonate concentration profile. In addition, effects of flow on passive bicarbonate diffusion or convection could not account for the observed stimulation. When tubules were perfused with 58 mM bicarbonate (a concentration previously shown to achieve maximal rates of proton secretion), increasing flow rate from 15 to 49 nl/min did not stimulate bicarbonate absorption. Thus, when examined as a function of mean luminal bicarbonate concentration, increases in flow increased the rate of proton secretion without affecting the maximal rate. The data are most consistent with flow-dependent stimulation of bicarbonate absorption, secondary to flow-dependent changes in luminal bicarbonate concentration, occurring by two mechanisms: 1) flow-dependent increases in the measured axial luminal bicarbonate concentration profile and 2) flow-dependent decreases in radial luminal bicarbonate concentration gradients.