We investigated the effect of the calcium channel blocker verapamil on intraocular pressure in human volunteers. In the initial trial, 15 subjects with untreated ocular hypertension were tested. After a baseline measurement was obtained with applanation tonometry, a 40-microliter drop of verapamil, 1.25 mg/ml, was instilled in one eye. After 30 minutes, a second reading was taken. In a subsequent trial of 12 untreated ocular hypertensive subjects, the duration of action was determined using the same dose and method of delivery. Results showed that verapamil elicited a mean +/- S.E.M. change in intraocular pressure of -3.8 +/- 0.900 mm Hg in the treated eye, and -1.6 +/- 0.400 mm Hg in the untreated eye. This reduction was statistically different in both eyes (treated eye, P = .0007; untreated eye, P = .005). This decrease in intraocular pressure remained statistically significant when compared to predrug baseline values for up to ten hours.