We transplanted mouse embryonic stem (ES) cells pre-differentiated on a PA6 feeder cell layer into the striatum of 6-hydroxydopamine hemi-lesioned adult rats and studied the fate of the grafted cells 1 and 5 weeks post-grafting. At both time points, ES cell grafts contained tyrosine hydroxylase positive (TH+) and 5-HT immunoreactive cells. Between 1 and 5 weeks, there was an enlargement of the grafts and an increase in number of TH+ cells although the differences between the two time points were not significant. The mean number of TH+ neurons per striatum was 330 +/- 73 after 1 week and 1220 +/- 400 after 5 weeks. Over the same time period, mean soma profile area of the TH+ neurons increased significantly by 25.2%. Neurites were longer after 5 weeks (by 24.9%), but the difference to 1 week post-grafting was not reliable. The percentage of TH+ somata without neurites increased from 6.7% after 1 week to 38.3% after 5 weeks (not significant). After 5 weeks, two out of fifteen graft recipients had tumors indicating that pre-differentiation of mouse embryonic stem cells using this differentiation protocol is not sufficient to prevent tumor formation.