There is increasing evidence that herpes simplex virus (HSV) may reach the central nervous system along peripheral nerves (Johnson, 1964; Wildy, 1967). Johnson showed that after subcutaneous or intranasal inoculation of mice with HSV, there was spread of virus along corresponding cranial and peripheral nerves. The spread was thought to result from the centripetal infection of endoneural (Schwann and/or fibroblast) cells. Johnson was unable to demonstrate antigen inside axons by means of immunofluoresence, even when the corresponding ganglion cell perikaryon was infected. However, the recent work of Kristensson, Lycke & Sjöstrand (1971a) gives further weight to the much older hypothesis that the most important mode of spread is inside the axons (Goodpasture & Teague, 1923). Using 3-week-old suckling mice, inoculated intradermally in the hind footpad, the appearance of virus could be demonstrated in dorsal-root ganglion cells within 3 days of inoculation (Kristensson et al. 1971a; Wildy, 1967).