Certain biological membranes, such as the erythrocyte plasma membrane, have a high osmotic water permeability, and such membranes have long been suspected of harboring water channels. The molecular identity of these channels has now been established with the purification of water-channel proteins and the cloning of the genes encoding them. Homologous water-channel proteins, called 'aquaporins', are present in plants and animals. These channels are water selective and do not allow ions or metabolites to pass through them. Their discovery is providing new insights into how plant and animal cells facilitate and regulate the passage of water through their membranes.