Neuronal cell populations giving rise to cerebellar projections in the turtle, Pseudemys scripta elegans, were analysed following injections of horseradish peroxidase into the cerebellar cortex. The most prominent retrograde cell labeling occurred bilaterally within the caudal rhombencephalon and especially in the ventral portion of the inferior reticular field. Based on the structural parameters of the labeled cells (size, dendritic tree), their location and laterality of projection, attempts were made to identify cell groups similar to the inferior olive, the lateral funicular (reticular) nucleus and the perihypoglossal complex of other vertebrates. There were some labeled neurons within the descending and principal trigeminal nuclei, but few if any within the dorsal column nuclear complex. Cerebellar projections on the other hand clearly arose from the n.vestibularis inferior and n.vestibularis dorsolateralis on both sides. While there was little evidence for labeled cells located in a similar position as the pontine nuclei of higher vertebrates, a conspicuous number of neurons were observed in meso-diencephalic regions. Confirming the findings of Reiner and Karten (1978) characteristic accumulations of cells were seen in the nucleus opticus tegmenti, in the ipsilateral mesencephalic tegmentum and lateral and ventral to the ipsilateral nucleus pretectalis. Additional neurons were found in the periventricular hypothalamus, the nucleus of the fasciculus longitudinalis medialis and in the n.interstitialis of flm on both sides as well as in the red nucleus.
