To examine the mechanisms of cell volume regulation in response to hyperosmolality, segments of the inner stripe of rabbit outer medullary collecting duct (OMCDi) were perfused in vitro. The cross-sectional area of the tubule was monitored as an index of the relative cell volume. When luminal and basolateral osmolalities were increased from 290 to 390 mOsm simultaneously, the tubule cell shrank instantaneously and reswelled gradually, showing the so-called regulatory volume increase (RVI). Basolateral Na+ removal and addition of basolateral ethyl isopropyl amiloride (EIPA) decreased the RVI response by 76 and 66%, respectively. By contrast, apical Na+ removal had no effect on this response. RVI response was also inhibited by basolateral, but not luminal, Cl- removal (-63%), by total HCO3- removal (-74%), and by adding basolateral 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (-62%). Intracellular pH did not change significantly during RVI. Vasopressin increased RVI response by 56%. However, this increase was abolished in the absence of basolateral Na+ and Cl-, and in the presence of basolateral EIPA and DIDS. These results suggest that major mechanisms responsible for RVI are Na(+)-H+ and Cl(-)-HCO3- exchange systems in the basolateral membrane, and that these systems are stimulated by vasopressin in rabbit OMCDi.