Until recently, the mouse has been the animal of choice for transgenie studies. This is because the scientists who pioneered transgenic technologies emerged from the fields of mouse genetics and embryology. The mouse will continue to be the most convenient mammalian model for asking developmental, genetic, and immunological questions. Basic cancer studies will also continue to use the mouse as its principal animal model. However, in fields such as physiology and neurobiology, the mouse is a somewhat inconvenient system and, for scientists carrying out studies in these areas, it is the rat that has for many years provided the most appropriate model. The recent development of techniques that allow for the generation of transgenic rats will allow the application of reverse genetic techniques to physiological and neurobiological problems (1 ).