INTRODUCTION Although attempts to transplant neural tissue into the brain go back to the last century (Thompson, 1890), the modern period of intense investigation of such transplants has a very short history, dating from the pioneering experiments in Sweden of Bjorklund (Stenevi, Bjorklund, & Svengaard, 1976) and Olson (Olson, Seiger, Freedman, & Hoffer, 1980), and of Lund (Lund & Hauschka, 1976) in the USA. This history has been characterised by an unusually close partnership between neuroanatomists and psychologists, with other neuroscientists (notably, the neurophysiologists) rather lagging behind. In consequence, it rapidly became clear that neural transplants are capable both of surviving and flourishing in the host brain and of exerting powerful functional (i.e. behavioural) effects (Dunnett, Bjorklund, Stenevi, & Iversen, 1981), while the mechanism(s) by which these functional effects are exerted has remained in most cases

mysterious (Bjorklund & Stenevi, 1984; Dunnett, 1990; Gray, Sinden, & Hodges, 1990).