Reactive oxygen species (ROS) is the common name used to describe the partially reduced forms of molecular oxygen that may be generated in cells during oxidative metabolism. They are normally considered to be toxic, and cells possess various defence systems to protect themselves including antioxidant enzymes and low molecular weight antioxidants like vitamin C and vitamin E. However, it is now clear that small amounts of ROS also act as messenger molecules in cell signal transduction pathways; the plasma membrane of eukaryotic cells in particular contains a variety of different ROS-producing oxidases and reductases, of which the best characterized are the superoxide-producing NADPH oxidases. It has been known for many years that membrane redox activity can be changed rapidly by various hormones and growth factors, but the molecular mechanisms involved and the physiological importance of this phenomenon have only recently begun to be unveiled. This review summarizes the state of the art on plasma membrane-based ROS signalling in the pathways of insulin, steroid and thyroid hormones and growth factors. The apparent paradox of ROS being essential biomolecules in the regulation of cellular functions, but also toxic by-products of metabolism, may be important for the pharmacological application of natural and synthetic antioxidants.