Nerve cells positive for NADPH diaphorase (D)/nitric oxide synthase in the human ciliary muscle appear to be involved in relaxation of the muscle during disaccommodation. To study whether similar cells might mediate disaccommodation of the primate ciliary muscle in general, serial sections of the ciliary muscle of 5 cynomolgus monkeys (Macaca fascicularis) and 2 owl monkeys (Aotes trivirgatus) were stained for NADPH-D. Both monkey species have a ciliary muscle system and an accommodative amplitude comparable to that in humans. Positively stained cells were frequently observed in the ciliary muscle of all cynomolgus monkeys, a diurnal species with a fovea, but never in owl monkeys, a nocturnal species without a fovea. The results indicate that NADPH-D-positive and probably nitrergic ganglion cells in the ciliary muscle are not present in all primate species, but only in those with high requirements for visual acuity. They might smoothen the ciliary-ganglion-mediated contraction of the ciliary muscle or contribute to the small fluctuations or oscillations of accommodation that are observed under steady viewing conditions.