Vasopressin-regulated water permeability of the kidney collecting duct is a key component of the urine concentration machinery. Recently, a cDNA for AQP-CD, the vasopressin-regulated water channel, initially reported as WCH-CD, has been isolated (K. Fushimi, S. Uchida, Y. Hara, Y. Hirata, F. Marumo, and S. Sasaki. Nature Lond. 361: 549-552, 1993). AQP-CD was expressed in oocyte membrane using a Xenopus expression vector, and functional characteristics of AQP-CD were examined. Osmotic water permeability (Pf) of oocytes expressing AQP-CD was 138 +/- 19 microns/s (mean +/- SE), 12 times greater than the control (11 +/- 3 microns/s), 90% inhibited by 0.3 mM HgCl2, and weakly temperature dependent (energy of activation for Pf was 4.0 kcal/mol). Urea influx measured from 15-min [14C]urea uptake by oocytes injected with AQP-CD/expression vector 1 cRNA was 86 +/- 17% of the control. Two-electrode voltage-clamp experiments revealed insignificant ion conductance of AQP-CD. Immunoblots of membranes from rat kidney medulla and oocytes expressing AQP-CD using anti-AQP-CD COOH-terminal antibody showed a 29-kDa protein and 35- to 50-kDa high-molecular-mass forms. Immunohistochemistry showed apical and subapical localization of AQP-CD in the collecting duct principal cells. Our results indicated that AQP-CD is a 29-kDa protein, a selective water channel, distinct from a urea channel, and localized to the membranes of vasopressin-sensitive components in kidney collecting duct principal cells.