Horseradish peroxidase (HRP) was injected into the anterior eye chamber of rats and mice and frozen sections from both superior cervical and trigeminal ganglia were incubated to demonstrate neurons accumulating the tracer by retrograde axonal transport. Labelled cells were observed only in ganglia ipsilateral to the HRP injection. Within the trigeminal ganglion, peroxidase-containing neurons were restricted to the medial ophthalmic area, whereas labelled cells in the superior cervical ganglion were more widely distributed. With the use of a new and more sensitive technique for the demonstration of HRP in neurons, it was possible to show retrograde transport also of small amounts of peroxidase injected into the anterior eye chamber. In addition, this technique enabled identification of the central and peripheral processes of neurons in the trigeminal ganglion and the dendrites and axons of sympathetic ganglion cells. The rate of retrograde HRP transport in rats was calculated to approximately 4--5 mm/h for both sensory and adrenergic nerves, which is consistent with previous estimates for this protein. It differs from the transport rate reported for nerve growth factor (NGF) and macromolecular toxins in sensory and adrenergic nerves of the same species. These rates were, however, obtained with a different method and in a different population of sensory neurons and are, therefore, not directly comparable. After treatment with 6-hydroxydopamine the number of HRP-labelled cells in the superior cervical ganglion was significantly reduced compared to controls. Cell counts from trigeminal ganglia showed no significant difference between controls and treated animals.