In previous studies, when rabbit cortical collecting ducts were studied under standard conditions in vitro, bicarbonate transport was conditioned by the acid-base status in vivo of the animals from which the tubules were obtained. The collecting ducts absorbed bicarbonate if the animals were acidotic (from ammonium chloride administration), but they secreted bicarbonate if the animals were alkalotic (from sodium bicarbonate administration). The purpose of the present studies was to test whether similar conditioning occurs in other nephron segments. In proximal straight tubules neither the sodium-dependent nor the sodium-independent component of bicarbonate absorption was significantly affected by the acid-base status of the animals from which the tubules were obtained. .In cortical thick ascending limbs, chloride was absorbed, but there was no net transport of bicarbonate by tubules whether they came from acidotic or alkalotic animals. Since cortical thick ascending limbs were not found to transport bicarbonate, they cannot be implicated in the control of bicarbonate excretion. We conclude that the previously observed conditioning of bicarbonate transport occurs in collecting ducts but not in proximal straight tubules or in cortical thick ascending limbs in the same experimental situation.