Mongolian gerbils were enucleated, and a crystal of HRP placed on the cut surface of the transected optic nerves emerging from the eyeballs. After incubation in an oxygenated medium, glutaraldehyde fixation, cryo-sectioning and reaction of the sections for peroxidase activity, HRP-labelled fibres were observed in the optic nerve fibre layer of the retina. Some of the labelled fibres penetrated in the external direction from the ganglion cell layer into outer retinal layers. HRP-labelled fibres were observed in the inner plexiform layer, inner nuclear layer, as well as the outer plexiform layer. In the outer plexiform layer arborization of the fibres was prominent especially close to the outer nuclear layer. In all layers some fibres were beaded. Due to the lack of trans-synaptic transport by use of this in vitro method the study strongly indicates the presence of an efferent innervation of the retina of the Mongolian gerbil. A preliminary report on this research has been presented at the Tenth European Neuroscience Meeting, Marseille (1986).