The field of neural transplantation has rapidly progressed during the past two decades. Since the first published observations on the functional effects of transplanted fetal dopamine (DA) neurons in rodents (Perlow et al., 1979 ; Bj�rklund and Stenevi, 1979 ), there are now several ongoing clinical trials to assess the efficacy of transplanting human fetal neurons for the treatment of patients with Parkinson’s disease (PD) (Freed et al., 1992 ; Widner et al., 1992 ) and Huntington’s disease (HD) (Kopyov et al., 1996 ). However, a major limitation to the widespread clinical application of this approach is the inability to obtain consistently suitable specimens for transplantation, and the ethical concerns of using human fetal tissue. Consequently, the use of donor tissue from other species as xenografts has been proposed as an alternative to the use of human fetal allografts.