AbstractA commonly used method of evaluating critical flicker fusion (CFF) threshold—the Leeds psychomotor tester—has been assessed with regard to the influence of pupil‐size alteration on CFF measurements. Six healthy volunteers, (three males, three females) took part and were treated with saline, pilocarpine 2%, and homatropine 1% eye drops at intervals of one week according to a balanced crossover design. The regression of CFF threshold on pupil size, both measured up to 7 hr after treatment, was highly significant in all subjects. Increases of pupil size from 2 to 8 mm produced a rise in CFF threshold which varied between individuals but ranged from 2 to 9 Hz. In a separate study, the effect of clobazam 20 mg on binocular and monocular CFF, the latter both uncorrected and corrected for pupil size, was assessed over the 8‐hr period after treatment. No significant changes in either measure of CFF were observed. On a separate study day the same four subjects submitted to mydriasis/miosis induced by homatropine followed by pilocarpine eye drops. This procedure showed that the use of an artificial pupil of 2 mm adequately corrected for pupil‐size influences on CFF in all but on