Roller tube cultures of parasagittal cerebellar slices were taken from young rats aged 9-11 days, and maintained in vitro for 1-2 weeks. Morphological aspects of cell types and synaptic relationships in such organ cultures were examined at light and electron microscopic levels. Some neurons were marked by intracellular injections of horseradish peroxidase for subsequent identification of their connection patterns. Cytoarchitecture of the cerebellar cortex was largely preserved in the organ cultures. Dendritic trees of Purkinje cells exhibited isoplanar organizations that often resembled their orientation at the time of explanation. Other cerebellar neurons, namely granule cells, Golgi cells, basket cells, stellate cells, all differentiated within the organ cultures. In addition, some neurons of the deep cerebellar nuclei remained viable during the period of culture. Mossy fibers most probably of cerebellar nuclear origin were found terminating on the dendrites of granule cells and Golgi cells. Quite unexpected were certain types of direct synapses of afferent fibers on short necked spines arising from Purkinje cell smooth dendrites and somata. Such terminals resembled climbing fibers. They were most likely modified mossy fiber afferents, since the organ cultures did not include neurons of the inferior olive which are well spearated from the cerebellar mass at postnatal stages. These "ascending" mossy fibers presumably occupied postsynaptic surfaces that were either vacated by deafferentation or induced by the afferent fibers themselves. Intracellularly labeled Purkinje cells had widely distributed axonal collateral branches. Labeled axons were distributed within the Purkinje cell layer. Several recurrent Purkinje cell axon collaterals stained with reaction products of horseradish peroxidase tracer were followed at the ultrastructural level. In one case, labeled terminals were examined in an area of approximately 2 mm2. Terminals of Purkinje cell collaterals formed symmetric synapses with somata of basket cells and dendrites of Golgi cells, but not Purkinje cell somata. Some large boutons of serially traced Purkinje cell axon collaterals formed asymmetric contacts with profiles interpreted as Golgi cell dendrites. In contrast to the apparent axonal sprouting in cerebellar organ cultures, maturation of dendritic processes remained static. Astroglia cells of diverse shapes were observed following immunocytochemical staining with antisera to glia filament proteins. The distribution patterns of immunoreactive astrocytes changed dramatically in cerebellar slice cultures maintained for 3-6 weeks in vitro.