Studies of ammonia and bicarbonate transport are being conducted in isolated, perfused tubules from rats and rabbits. The results in rabbit cortical collecting duct showed that: (1) bicarbonate secretion occurs by electroneutral exchange for chloride; (2) bicarbonate secretion is controlled in response to the systemic acid-base state of the animals; (3) proton secretion occurs in parallel to bicarbonate secretion generating an acidic pH disequilibrium in the lumen; and (4) the luminal pH disequilibrium is responsible for ammonia secretion by nonionic diffusion. The results in the rat cortical collecting ducts were qualitatively the same except that the presence of 4 mM total ammonia in perfusate and bath converted bicarbonate secretion to absorption. In the rabbit proximal straight tubule, ammonium was found to substitute for potassium in supporting fluid and bicarbonate absorption indicating that ammonium is able to exchange for sodium on the Na-K-ATPase.