The ciliary muscle from 44 rhesus monkeys ranging in age from 137-day fetuses and a 3-week neonate to 35-year-old adults was studied by light and electron microscopy. In the fetuses and the neonate, the muscle cells appeared immature, and the muscle consisted mainly of longitudinally oriented fibers. By age 1 year, the muscle fibers were aggregated into bundles, and within each bundle the individual fibers were packed tightly together. In sagittal section, the muscle now exhibited longitudinal, reticular and circular portions. The individual muscle cells exhibited ultrastructural characteristics which set them apart from other smooth muscle cells. Age-related structural changes occurred in the ciliary muscle and intramuscular nerves, beginning at 6 years. These included increasing numbers of lysosomes, 'fingerprints', and the appearance of myelin figures within some nerve endings and nerve fibers. These alterations became more frequent and pronounced with age, and in elderly animals were seen in all regions of the muscle and around its entire circumference. Additionally, the elderly animals exhibited overt degeneration of some muscle cells and some myelinated nerve fibers. The time-course of these age-related structural neuromuscular alterations parallels the decline of functional accommodative amplitude (i.e. presbyopia) and of the ciliary muscle's configurational response to peripheral pharmacologic and central electrical stimulation. This suggests a pathophysiologic, albeit (given the relatively moderate overall abnormalities) not necessarily causal, relationship. An age-related increase in the number of pigmented cells between the ciliary muscle bundles also occurred, with the anterior longitudinal region being affected last. This might reflect an age-related decline in uveoscleral drainage of aqueous humor.