We describe here a culture system for long-term growth of organotypic slices of spinal cord, with attached dorsal root ganglia (DRG) derived from 13-14 day mouse fetuses. This is a unique in vitro tool in which both central and peripheral nervous tissue grow and differentiate in culture to become heavily myelinated. During cultivation the slices and the ganglia become flattened so as to allow microscopical and immunocytochemical staining. When both central and peripheral myelin had been formed (usually around the third week of cultivation), cultures were infected with 5 x 10(6) PFU of West Nile Virus (WNV). Progeny virions appeared first in about 10% of the neurons and were subsequently observed between lamellae in the central myelin sheath of several axons. Such viral arrangement in relation to myelin membranes, might provide a novel concept for a possible mechanism underlying slow viral infection.