We recorded pupillary responses to alternating half-field visual stimuli in 13 normal subjects (eight men and five women ranging in age from 23 to 28 years). We found that stimulation of the temporal visual field caused more pupillary constriction than did equivalent stimulation of the nasal field. Temporal stimuli also produced a direct pupillary response that was larger than the consensual response. Nasal stimulation, however, elicited a larger consensual pupillary response than a direct response. Our findings imply an asymmetric decussation of pupillomotor pathways in the midbrain in humans, as well as greater sensitivity of the nasal retina.