PURPOSE: Determine the effects of supraclinical topical doses of pilocarpine on the structure of the trabecular meshwork (TM), ciliary muscle (CM), and ciliary processes in monkeys. METHODS: Cynomolgus monkeys received topical pilocarpine hydrochloride doses of 1 to 2 mg unilaterally once or twice daily for 1 to 211 days. Both eyes were examined by light and electron microscopy. RESULTS: After 1 dose, the TM was expanded and the flow pathways open. Some elastic anterior CM tendons were disconnected from the muscle tips and/or the tips themselves were disrupted; macrophage-like cells accumulated in these regions. In eyes that had long-term treatment, scar tissue at the muscle tips and disruption of myofibrils within muscle cells indicated regions of previous disconnection. Most collagen tendons inserting into the peripheral cornea were unaffected. With increasing treatment duration in younger monkeys, there was anteroposterior shortening of Schlemm's canal, TM densification due to collapse, and increased cellularity in the subendothelial region of Schlemm's canal. TM densification and Schlemm's canal shortening were not found in older monkeys, implicating posterior movement of the elastic young limbal region induced by strong CM contraction, rather than pilocarpine toxicity. In the ciliary body we found vasodilation, Greeff's vesicles, and blood-aqueous barrier breakdown in the anterior ciliary processes and vasodilation, edema, and round-cell infiltration in the posterior pars plana. CONCLUSIONS: Large pilocarpine doses induce morphologic changes in the monkey anterior segment, likely consequent to intense CM contraction. Changes in the TM and Schlemm's canal are seen only in young monkeys, apparently owing to mechanical deformation of the elastic limbal region.