Superior cervical ganglionectomy causes a very rapid and complete distintegration of adrenergic axons in the rat iris commencing less than 24 hours after operation. At 48 hours post-operatively the electronmicroscopical appearance of the iris is normal with osmium tetroxide fixation because the debris has vanished and the abundant surviving cholinergic axons in the autonomic ground plexus give the impression that the total innervation is still intact. However, with permanganate fixation the denervated iris examined 48 hours to one year after operation lacks granular vesicles in its axons and appears in this single respect different from normal. After ciliary ganglionectomy the same kind of degeneration of individual axons begins within 24 hours, but continues for several days, involving a greater number of axons than the previous operation. Removal of both sets of ganglia intensifies the loss of axons and causes some atrophy of the iris tissues. A residue of apparently intact axons persists, however, for as long as ten months. The previously discordant results of ganglionectomy on iris innervation obtained by light microscopy are shown to be dependent on technical inadequacies, combined with the fact that in the earlier literature it was not known that there is a close intermingling of adrenergic and cholinergic axons in all parts of the iris plexuses.