In vivo tubular microperfusion experiments were performed in rats to compare the function of the early and late rat distal convoluted tubule and to examine the effect of amiloride on distal Na and Ca reabsorption. In clearance experiments, amiloride (6 micrograms/min) reduced fractional Ca excretion (P less than 0.05) and the calcium-to-sodium clearance ratio (P less than 0.001) without affecting fractional Na excretion. The in vivo microperfusion experiments revealed a higher rate of osmotic water flow in late distal convoluted tubules than in early tubules (P less than 0.001), while Na and Ca transport rates were comparable. The addition of 10(-5) M amiloride to the luminal perfusate inhibited net fluid reabsorption (P less than 0.05) and Na reabsorption (P less than 0.01) and enhanced Ca reabsorption (P less than 0.005) in late segments of the distal tubule. The drug did not change early distal function. The degree of enhancement of Ca transport was highly correlated with the degree of inhibition of Na transport, suggesting that the two effects are related. We suggest that increased Ca reabsorption is a result of luminal membrane hyperpolarization or of increased rates of Ca-Na exchange.