Anatomical and physiological studies in adult rats following neonatal hemicerebellectomy reveal that the intact half of the cerebellum is capable of remodeling its connections to the ipsilateral thalamus. The present study investigates to what extent additional cerebellar neurons project to the ipsilateral thalamus in the remodeled pathway using a retrograde horseradish peroxidase (HRP) technique. Left hemicerebellectomies were performed on Sprague-Dawley rats within 24 h after birth. An injection of HRP was made into the right thalamus when the animals reached adult size. Unoperated control animals were similarly injected with HRP. Labeled cells were counted in the following ipsilateral cerebellar nuclei: fastigial nucleus (FN), anterior interpositus nucleus (AIN), posterior interpositus nucleus (PIN), dorsal lateral hump (DLH) and dentate nucleus (DN). Total cell counts revealed a 4-fold increase in the number of labeled cells in the experimental group vs the control group. Significant increases in the number of labeled cells were observed in the AIN, PIN, DLH and DN; however, the number of labeled cells in the FN did not differ between the two groups. Comparison of the proportional amount of labeling, relative to each nucleus, indicates that there is a decrease in the FN and an increase in the AIN, PIN, DLH and DN. These numerical data are supported by statistical analysis (Mann-Whitney U-test and Student's t-test, P less than 0.05). The data are supportive of the original hypothesis that the ipsilateral cerebellothalamic projection increases following neonatal hemicerebellectomy. The results also suggest that the reorganized pathway attempts to mirror the normal contralateral cerebellothalamic projection.