Ibotenic acid was injected unilaterally into the baboon caudate-putamen (CP) to achieve a neural degeneration model in the primate, with a neuropathology similar to Huntington's disease. Four to six weeks later injections of cell suspensions of striatal precursor cells, obtained by dissection of the fetal rat striatal region (13-15 days gestational age), were made into the excitotoxically lesioned CP of 3 baboons immunosuppressed by Cyclosporin A. Morphological analysis indicated that in one of the baboons, which had the largest lesion of the CP and the shortest survival time (6 weeks after implantation), there was a surviving striatal implant. The implanted neurons grew in high densities in cellular aggregates within the host gliotic CP. These neurons had a neuronal size phenotypical for rat striatum, i.e. on average about a 25% smaller neuronal cell diameter than a similar population in the baboon caudate-putamen. Glial-fibrillary-acid-protein immunoreactivity was present on large astrocytes within the striatal implant, with a distinct border towards the lesion-induced astrogliosis of the host. Neuronal markers for acetylcholinesterase and Leu-enkephalin were distributed in a typical patchy manner in the striatal implants along with fiber staining for tyrosine-hydroxylase-like immunoreactivity (TH) possibly derived from afferent host dopaminergic axons. Some of these fibers in the implants came from intrinsic TH-positive neuronal somata, probably of neocortical fetal origin and transiently expressing the enzyme. In conclusion, the results indicate that neuronal replacement can be achieved by cross-species implantation of fetal striatal precursor cells to the previously neuron depleted primate CP under immunosuppression but that the survival and growth of such implants may be variable and subject to unfavourable trophic conditions.