The CNS of the newly born opossum removed in its entirety survives and maintains its electrical excitability in suitable culture media for up to ten days at 25 degrees C. The structure of the developing neonatal spinal cord has been studied in the intact animal and in the cultured CNS. The differentiation and survival of individual cells and subcellular structures were followed at the light and electron microscopic level. The expression of cell markers in neuronal and glial cells was studied immunocytochemically using commercially available antibodies. Both mono- and polyclonal antibodies raised against antigens from several other species cross-reacted with Monodelphis antigens. The spinal cord of preparations removed from three-day-old-animals showed many neuron specific enolase-positive large neurons in the ventral horn as well as vimentin- and glial fibrillary acidic protein-positive radial glial cells and numerous small diameter unmyelinated axons, abundant dendrites and synaptic structures. From post natal day 5 to post natal day 8 continued differentiation of neurons and differentiation of radial glial cells into astrocytes were apparent. Radial glial fibres and astrocytes reacted positively to antibodies against glial fibrillary acidic protein. Myelin had not appeared at 8 days. A comparison of material obtained from postnatal day 3-postnatal day 4 preparations fixed immediately after dissection and from postnatal day 3-postnatal day 4 preparations fixed after 5 days in culture showed growth with continued mitotic activity of the neuroepithelial cells and further neuronal and glial maturation in the spinal cord especially in the more rostral end. In successful experiments in vitro, the preservation of individual cells, organelles, membranes and synapses was similar in the freshly dissected and cultured preparations apart from a distinct loss of the youngest and some of the oldest neurons in the spinal cord. Also the main fibre tracts (dorsal, lateral and ventromedial funiculus) survived. Virtually all preparations that had not been damaged or injured showed these results. Possible reasons for the death or survival of individual neuronal or glial cell populations in these preparations are discussed.