Adult female cats (2.4-2.8 kg) underwent surgical removal of the left ciliary ganglion under pentobarbital anesthesia. Twenty-one, 560 and 616 days later, pupil size of both left and right (control) eyes was measured in response to progressively increasing doses of carbachol applied topically. By 21 days after denervation, ganglionectomized eyes showed marked supersensitivity to the miotic effects of pilocarpine and carbachol. By 560 days, however, responsiveness of the denervated eyes to lower and intermediate doses of carbachol was the same as that of contralateral control eyes, while responsiveness to higher doses was significantly reduced. Responsiveness to both lower and higher doses of carbachol was significantly less than that of the controls on the 616th day. Ganglionectomized eyes showed no pupillary response to light 14, 562, or 620 days after surgery. Histochemical analysis of iris tissue collected from eyes of these cats 720 days after ganglion removal revealed an almost complete absence of acetylcholinesterase-containing nerve fibers on the denervated side. These findings indicate that the return to normal or lower sensitivity to carbachol of denervated eyes at prolonged periods after ciliary ganglion removal is not due to significant cholinergic reinnervation of the iris sphincter muscle.