1. The effect of purified bovine parathyroid hormone on renal tubular reabsorption of sodium and calcium has been studied by micropuncture in intact and recently thyroparathyroidectomized dogs. 2. Parathyroid hormone increased the rejection of sodium and calcium proportionately at the late proximal tubule in both intact and operated dogs. 3. In both groups of dogs, there was increased delivery of sodium and calcium to the distal tubule after the hormone. However, the Ca/Na ratio decreased, suggesting some selective enhancement of calcium reabsorption before the superficial distal puncture site. 4. In the final urine, the Ca/Na ratio decreased highly significantly in both groups of dogs, indicating a further selective effect of parathyroid hormone on calcium reabsorption in or beyond the distal convoluted tubule.