Calcium and sodium transport were studied in the distal convoluted tubule of the rat by micropuncture and microperfusion techniques. The animals received either control infusions, 0.9% saline at 0.02 ml/min, or chlorothiazide (CTZ), 0.5 mg/min-kg. In free-flow micropuncture, distal calcium reabsorption occurred against an electrochemical gradient; it was 9% of the filtered load in the controls and 13.8% in the CTZ-treated rats. The drug dissociated calcium and sodium transport along the distal tubule. Absolute reabsorptive rates for calcium and sodium, measured in pump perfusion experiments, were proportional to the distal loads. CTZ, acting from the lumen, enhanced net calcium (P less than 0.01) and reduced net sodium (P less than 0.01) reabsorption. In stationary microperfusion experiments, sodium backflux produced rapid establishment of a steady-state concentration of 50 mM. In contrast, net backflux of calcium was negligible. With calcium in the luminal perfusion fluid, there was a concentration-dependent reabsorptive efflux which was not saturated at a luminal calcium concentration of 9.5 mM and which was enhanced by CTZ.