Twenty-nine patients with presumed compressive optic neuropathy were studied, and pupil cycle times were measured. We found (1) normalcy of the test in cases of space-occupying lesion without suprasellar extension or after successful operation; (2) prolongation of pupil cycle time in cases of subclinical disorder conformed by visual evoked responses as well; (3) a good correlation between the values of prolonged pupil cycle time and degree of involvement of the visual field in pathological cases.