Aquaporin channel-forming integral protein (CHIP) is the first characterized water channel protein (genome symbol AQP1), but the molecular structure of the aqueous pathway through CHIP remains undefined. The two halves of CHIP are sequence-related, and each has three bilayer-spanning domains with the motif asparagine-proline-alanine (NPA) at residues 76-78 (in cytoplasmic loop B) and 192-194 (in extracellular loop E). The NPA motifs are oriented 180 degrees to each other, and the second NPA is near cysteine 189, the known site where mercurials inhibit osmotic water permeability (Pf). When expressed in Xenopus oocytes, the double mutant A73C/C189S exhibited high, mercurial-sensitive Pf similar to wild-type CHIP. Conservative substitutions of slightly greater mass in or near NPA motifs in loop B or loop E in CHIP caused reduced Pf and failure of the protein to localize at the plasma membrane. Certain nonfunctional loop E mutants complemented the truncation mutant D237Z. Formation of mixed oligomers was demonstrated by velocity sedimentation, immunoprecipitation, and analysis of dimeric-CHIP polypeptides. Cellular distributions of individual mutants or complementing pairs of mutants were verified by plasma membrane isolation and confocal microscopy. An hourglass structural model is proposed in which a cytoplasmic chamber (loop B) connects within the membrane to an extracellular chamber (loop E) forming a single, narrow aqueous pathway through each of the CHIP subunits; subunit oligomerization may provide the vertical symmetry necessary for residence within the lipid bilayer.