Water channels AQP7 and AQP8 may be involved in transcellular water movement in the small intestine. We show that both AQP7 and AQP8 mRNA are expressed in rat small intestine. Immunoblot and immunohistochemistry experiments demonstrate that AQP7 and AQP8 proteins are present in the apical brush border membrane of intestinal epithelial cells. We investigated the effect of several metals and pH on the osmotic water permeability (P f ) of brush border membrane vesicles (BBMVs) and of AQP7 and AQP8 expressed in a cell line. Hg 2+ , Cu 2+ , and Zn 2+ caused a significant decrease in the BBMV P f , whereas Ni 2+ and Li + had no effect. AQP8-transfected cells showed a reduction in P f in the presence of Hg 2+ and Cu 2+ , whereas AQP7-transfected cells were insensitive to all tested metals. The P f of both BBMVs and cells transfected with AQP7 and AQP8 was not affected by pH changes within the physiological range, and the P f of BBMVs alone was not affected by phlorizin or amiloride. Our results indicate that AQP7 and AQP8 may play a role in water movement via the apical domain of small intestine epithelial cells. AQP8 may contribute to the water-imbalance-related clinical symptoms apparent after ingestion of high doses of Hg 2+ and Cu 2+ .