The refractive power and axial dimensions of the eye were measured under resting and fully accommodated conditions in 123 caged rhesus monkeys ranging in age from 0.5 to greater than 30 years. The mean resting refraction measured under ketamine anesthesia was -5 diopters. Accommodative amplitude, calculated as the difference between resting refraction and the most negative refraction measured 0.5 to 1 hr after topical application of a maximally effective dose of a cholinomimetic, showed an age-dependent decline. The mean accommodative amplitude of 1- to 5-year-old rhesus monkeys was a remarkable 34 D, while animals over 25 years of age averaged 5 D of accommodation. Some greater than 25-year-old animals showed no measurable change in refraction regardless of the dose or the type of cholinomimetic (carbachol, pilocarpine, or echothiophate) used. The resting axial thickness of the lens was found to increase with age throughout adulthood, well past the end of the growth period. A strong correlation was found between pharmacologically induced change in the refractive power of the eye and change in lenticular thickness. These similarities to the human condition suggest that the rhesus monkey represents a highly suitable animal model for the study of accommodation and presbyopia.