Previous studies in living animals have suggested that variations in the serum concentrations of calcium and phosphate directly affect renal tubular bicarbonate transport. To examine this area more directly, we measured the bicarbonate and the fluid transport in isolated rabbit proximal convoluted and straight tubules perfused in vitro. The concentration of calcium in the perfusate was varied between 0 and 5 mM and in the bath between 0.5 and 5 mM. In proximal convoluted tubules, 5 mM calcium in the perfusate and bath caused a significant increase in the rate of bicarbonate absorption, and removal of calcium from the perfusate inhibited both bicarbonate and fluid absorption. The concentration of phosphate in the perfusate and the bath was varied from 0.25 to 5 mM. In both convoluted and straight tubules, there was no significant change in bicarbonate or fluid absorption with changes in the ambient phosphate concentration. We conclude that changes in calcium concentration have significant effects on bicarbonate transport by proximal convoluted tubules but that wide variations in the concentration of phosphate have no significant effect on bicarbonate, sodium, or fluid transport by proximal tubules in vitro.