Advances in determining the mechanisms that underlie the control of energy balance in mammals have recently been provided by the discovery and characterization of aquaporin-7 (AQP7), a water-glycerol transporter that is present in the plasma membrane of fat-storing cells (adipocytes). Recent studies have shown that the absence of AQP7 expression in fat cells increases glycerol kinase activity, boosting triacylglycerol synthesis and ultimately leading to obesity. Thus, AQP7 operates as a glycerol channel in vivo, whereby adipocyte glycerol permeability has a key role in the regulation of fat accumulation.