Aquaporins were first described as channels which increased permeability of lipid membranes to water, in response to osmotic and/or hydrostatic gradients. Some were shown to increase permeability also of other small solutes, particularly urea and glycerol. More recently other functions have been attributed to some, such as involvement in angiogenesis, cell migration, and adhesion. All of these processes are essential to the establishment and maintenance of a healthy placenta, fetus, fetal membranes and fluids. So far aquaporins have been found in the placenta and fetal membranes [AQPs1,3,4,5,8,9] and in organs producing fluid into the amniotic compartment [lung 1,3,4,5; kidney 1,2,3,4], and it is postulated that these aquaporins may function in all of the above roles in these organs. It is proposed the ontogeny of aquaporin expression in a species (sheep) in which lung and kidney mature in comparable fashion to the maturation of these organs in human are more relevant to the understanding of the potential importance of these channels in the human than studies in the short-gestation, relatively immature young of the rodents.