Horseradish peroxidase (HRP) was dripped on the scarified left cornea of adult mice. Twenty-four hours later the animals were fixed by vascular perfusion and frozen sections cut from both trigeminal ganglia. After incubation for peroxidase activity labelled nerve cells were restricted to the medial ophthalmic part of the ganglion ipsilateral to HRP administration. If the scarification was omitted no neuronal labelling was observed. This labelling of the neurons is most probably the result from axonal uptake and subsequent retrograde axonal transport of the tracer. The similarity in distribution of peroxidase labelled nerve cells and the first ganglionic lesions occurring after instillation of herpes simplex virus in the cornea is pointed out.