The fine structure of reaggregating cultures of cells from 6- to 7-day-old mouse cerebellum was studied at intervals between 3 and 21 days in vitro (DIV). The resulting aggregates consisted mainly of small neurons (granule, stellate and basket cells), neuroglial cells and their processes. Large neurons were rarely present. By 7 DIV the previously loosely packed components had tightened into a more compact mass. A peripheral plexiform layer had formed which had many fine axons arranged into fascicles of parallel fibers. Deep to this zone was a cellular region containing clusters of neurons interspersed with small areas of neuropil. Axosomatic synapses appeared on neurons which resembled stellate or basket cells but not on granule cells. Axo-dendritic synapses formed in the neuropil of the cellular zone and, less frequently, in the outer plexiform layer. After 3 weeks glial cell processes had increased in volume at the expense of neurons. When cerebellar cells were cultured with cells from pons and medulla, which are normal sources of mossy fiber input, aggregates formed in which synaptic glomeruli were found. They were not seen in aggregates containing cells from retina and olfactory bulb cultured with cerebellum. Our observations suggest: that natural histogenetic mechanisms persist after dissociation and reaggregation of cerebellar cells resulting in a separation of an outer, 'molecular'-like layer from an inner granule cell layer and that neurons retain specificity of their synaptogenic capabilities both with regard to appropriate cell types and the morphological form that the synapses take.