AbstractThe present study analyzed whether grafts of the mesencephalic progenitor cell line CSM14.1 into the neonatal rat caudate putamen (CPu) differentiate into neurons and whether this is accompanied by a functional improvement in 6‐hydroxydopamine (6‐OHDA)‐lesioned animals. As in previous studies, a neuronal differentiation of CSM14.1 cells transplanted into the CPu of adult animals could not be observed, so we here used neonatal rats, because graft location and host age seemingly are crucial parameters for neural transplant differentiation and integration. Rats bilaterally lesioned at postnatal day 1 by intraventricular 6‐OHDA‐injections 2 days later received 100,000 CSM14.1 cells prelabelled with the fluorescent dye PKH26 into the right CPu. Five weeks after grafting, the cylinder test was performed, and the data compared with data from age‐matched intact controls and bilaterally lesioned‐only animals. Brain slices immunostained for tyrosine hydroxylase (TH) were quantified by optical densitometry. We observed a significant preference of left forelimb use exclusively in transplanted animals. In these rats, TH‐containing perikarya were found in the grafted CPu, presumedly leading to the significant increase of TH‐immunoreactive fibers in this region. Moreover, confocal laser microscopy revealed a differentiation of transplanted PKH26‐labelled CSM14.1 cells into neuronal nuclei antigen or TH‐immunoreactive cells. Thus, CSM14.1 cells differentiate into TH‐containing neurons, which most probably contribute to the preferred forelimb use, indicating a functional integration of CSM14.1 cells into the host basal ganglia loops during early postnatal development. These findings that are in contrast to observations in adult rats suggest instructive cues for neuronal differentiation and integration given by the neonatal microenvironment. © 2007 Wiley‐Liss, Inc.