Many studies of the pupillary light reflex pathway in mammals have indicated that the pretectum is important for this reflex. However, no single retinorecipient pretectal nucleus has been unequivocally identified as being involved in the light reflex pathway. In this study, anatomical studies in the rhesus monkey were carried out to identify the relevant retinorecipient pretectal nucleus and to better define the central pathway of this reflex.An injection of Wheatgerm Agglutinin/Horseradish peroxidase, a neuroanatomical tracer, was placed under physiological guidance into the Edinger-Westphal nucleus. Intravitreal injection of the same tracer in another animal was used to define the pretectal retinal terminal fields.Following injection of tracer in the Edinger-Westphal nucleus, retrogradely labeled cells were found in only one retinorecipient nucleus, the pretectal olivary nucleus. Most labeled cells were located contralateral to the injection site. A few labeled cells were located ipsilaterally. Intravitreal injection of tracer resulted in anterograde labeling of all the retinorecipient pretectal nuclei, including the pretectal olivary nucleus. The retinal terminal field in the pretectal olivary nucleus coincided with the location of the cells that were retrogradely labeled by the injection of tracer into the Edinger-Westphal nucleus.These results demonstrate that there is a direct projection from the pretectum to the Edinger-Westphal nucleus, that it arises from only one retinorecipient pretectal nucleus, the pretectal olivary nucleus, and that cells in the pretectal olivary nucleus almost all appear to project to the contralateral Edinger-Westphal nucleus.