The mydriatic effects of mepenzolate, a quarternary antimuscarinic agent, has been investigated and compared with those of atropine by pupillographic analysis in rabbits. The time-response curve for the onset of action for the agents (half-time, ca.5 min) was the same for the two agents despite the difference in structures. Although the half time for the offset of action (ca.18 and 34 hrs. for atropine and mepenzolate respectively) was different, the mydriatic effects of both agents were completely eliminated in 74 hrs.