Aquaporin 7 has greater sequence similarity with AQP3 and AQP9 and they may be a subfamily. Expression of rat AQP7 in Xenopus oocytes stimulated osmotic water permeability, as well as glycerol and urea transport. Characteristics of the water permeability of sperm of various vertebrate species were similar to those induced by AQP7: high water permeability with low activation energy, which was insensitive to mercury chloride. Northern blot analysis and in situ hybridization revealed that AQP7 is expressed abundantly in rat testis seminiferous tubules, in cells that appear to be late spermatids. This result was confirmed by immunohistochemistry, which showed that AQP7 is expressed during the late stages of spermatogenesis, and is localized on the plasma membrane of late spermatids.