The ophthalmic division of the trigeminal nerve was denervated stereotactically by electrocoagulation. Damage to the third nerve was avoided. Four days after denervation the homolateral iris was studied electron microscopically. There was Wallerian degeneration in the myelinated and also in part of the non-myelinated nerves of the iris. The ultrastructure of the iris nerves in the contralateral eye was normal. It is suggested that denervation of the ophthalmic division of the trigeminal nerve had caused interruption of the sympathetic nerves accompanying the trigeminal sensory nerves, and that the degenerated non-myelinated nerves of the iris are sympathetic in origin.