Telencephalic grafting represents a powerful tool for developmental studies and for the investigation of biological features of transgenic brain tissue. The interpretation of grafting experiments, however, requires detailed knowledge of graft biology. Therefore, we have characterized growth rates, graft size, and differentiation of embryonic telencephalic tissue harvested at various developmental stages and grafted into the caudoputamen and lateral ventricles of histocompatible mice. A total of 164 grafts were analysed up to 500 days after transplantation. Of all transplants, 79.3% resulted in the formation of solid neural grafts. Grafted cells were identified by 3H-thymidine labelling and autoradiography. Proliferation was studied by bromodeoxyuridine incorporation and decreased from an initial 35% at 1-3 d after grafting to less than 1.6% after 40 days. The graft size was measured as a function of the embryonic age of the transplanted tissue. Our data indicate that telencephalic tissue harvested at embryonic day E 12.5 reproducibly yields large, fully differentiated neuroectodermal grafts. The parameters defined in this study will be useful for detailed analysis of neuroectodermal tissue from mice undergoing fatal neurodegeneration, such as knockout mice bearing lethal mutations.