The stimulatory effect of cadmium ions on the Na-dependent fluorescein transport into the frog renal proximal tubules ceased with decreasing Ca++ concentration in solution on both the sides of the cell layer down to micromolar level. The decrease in Ca++ concentration per se stimulated fluorescein uptake during short-term incubations. A further diminution of Ca++ concentration in the tubular lumen with the aid of EGTA resulted in a sharp inhibition of the organic acid transport. Amiloride, which prevented the stimulatory effect of cadmium, inhibited the fluorescein transport at both millimolar and micromolar levels of Ca++ concentration, but it failed to affect the transport process after introducing EGTA into the tubular lumen. The results are discussed within the frames of a model regarding extracellular Ca++ as an allosteric inhibitor, and intracellular Ca++ as an allosteric activator of sodium channels in the apical membrane. Cd++ is assumed to compete with Ca++ for binding to centers of the allosteric inhibition, thereby accelerating the sodium ion flux across the cells of the proximal tubules.