Bicarbonate transport in the rabbit cortical collecting tubule (CCT) and outer medullary collecting tubule (MCT) in vitro was studied under two types of conditions that were anticipated to alter distal tubule bicarbonate transport: 1) reduction of renal mass, and 2) acid and base loading in vivo. Bicarbonate secretion (both total and acetazolamide sensitive) and bicarbonate reabsorption (studied separately) in CCT and bicarbonate reabsorption in the MCT were not different between tubules from normal and remnant kidneys. The control or conditioning of the separate processes of bicarbonate secretion and bicarbonate reabsorption was also studied in CCT from normal and remnant kidneys. Bicarbonate secretion was not increased by base-loading animals with either normal or remnant kidneys. In contrast, bicarbonate secretion was consistently decreased by acid loading (studied in CCT from remnant kidneys). Bicarbonate reabsorption in the CCT was not altered by acid or base loads given to animals with normal kidneys. And bicarbonate reabsorption in MCT was not increased by acid loading of animals with remnant kidneys. These studies demonstrate that bicarbonate transport (and its conditioning by acid or base loads in vivo) in both CCT and MCT in vitro is not altered by reduction of renal mass in rabbits. The predominant conditioning effect of acid or base loads in vivo is for acid loads to inhibit CCT bicarbonate secretion.