Purpose. The goal of this study was to investigate the relative contribution of parasympathetic and sympathetic it nervation of the iris musculature to pupillary dynamics, and to obtain a model of the rhesus pupil plant. Methods. Monkeys fixated a dim laser spot on a tangent screen while stimuli were presented to each eye in Maxwellian view. In two alert, tranned rhesus monkeys, single-unit recording was used to localize the intracranial port on of the oculomotor nerve close to its point of exit. Then the effect of brief electrical microstimulation (10-200 ms; 100 Hz; 75 μA) of the parasympathetic preganglionic fibers was compared to brief light pulses (10-200 ms; 1-1000 trolands) under normal conditions and with pharmacological sympathectomy produced by application of dapiprazole hydrochloride (0.5%) to one eye. Eye movements were measured using the scierai search coil technique, and the pupil diameter of each eye was measured using an ISCAN RK-406 pupillometry system. Results. The dynamics of the pupillary responses evoked by brief light flashes were very similar to those evoked by activation of the parasympathetic preganglionic fibers alone. The relationship between the pupillary responses elicited by light or by electrical microstimulation were essentially unaltered by pharmacological sympathectomy. While a second-order model of the plant could approximate these pupillary responses, it was found that a third-order model gave a more complete description. Conclusions. For brief light stimuli, parasympathetic innervation of the iris determines essentially the entire pupillary response.