In patients with unilateral Adie's syndrome, we compared the relative supersensitivity of the tonic pupil to methacholine hydrochloride (Mecholyl 2.5%) and pilocarpine 0.125%. We tested a large series of patients and photographed the pupils before and 30 minutes after the eyedrops. In a group of 36 patients, Mecholyl was positive in 64% of the tests; and in a group of 20 patients, pilocarpine 0.125% was positive in 80% of the tests. Eight patients who failed to show supersensitivity with Mecholyl were tested with pilocarpine and five of the eight were shown to be supersensitive. We concluded that pilocarpine, in this concentration is not only an adequate substitute for Mecholyl 2.5%, but is a more sensitive testing substance in unilateral cases, simply because it has a slightly stronger miotic action.