Using an open-loop stimulus (Maxwellian view), the direct pupillary response to light and its derivative curves (velocity and accleration of pupillary reactions) were recorded by infrared video pupillography in seven patients with myasthenia gravis and in three normal subjects. Responses before and after intravenous injection of 5 mg edrophonium hydrochloride were determined. Five of the patients had never taken anticholinesterase drugs except for diagnostic injections of edrophonium while the other two patients had been treated with oral preparations of corticosteroids. Analyses of the pupillary responses showed reduced amplitude, amximal velocity, and maximal acceleration of pupillary constriction in the myasthenic patients. On the other hand, changes in parameters of pupillary dilatation were minimal. Abnormal values returned towards normal within five minutes after edrophonium injection in all patients except in one individual who was being treated with corticosteroids at the time of examination. These results suggest that involvement of the iris sphincter may be common in patients with myasthenia gravis and this can be demonstrated by analysing the pupillary reaction and its derivative curves in response to an open-loop stimulus.