RETINAL ganglion cells of the fish have the spontaneous capacity to regenerate after nerve crush, a phenomenon known to be facilitated by nerve growth factor (NGF). We have studied the high-affinity NGF receptor TrkA, during the regeneration of the tench (Tinca tinca L.) optic nerve, using immunocytochemical techniques. TrkA-like immunoreactivity increased during the regeneration of the retinal ganglion cells. The increase is followed by a change in the subcellular distribution from perinuclear in control cells to cytoplasmic and perinuclear in regenerating ones. This increase was observed when antibodies against the extracellular domain of TrkA were used; no changes in TrkA-like immunoreactivity were observed with antibodies against the intracellular domain of TrkA. We thus conclude that modulation of TrkA is involved in the regeneration of fish retinal ganglion cells.