We evaluated the effect of the antihypertensive drug ketanserin, a 5-HT antagonist, on intraocular pressure (IOP) in 20 patients with ocular hypertension. IOP, pupil diameter, systolic arterial pressure (SBP), diastolic arterial pressure (DBP) and heart rate (HR) were recorded at baseline and at 1-hr intervals for 3 hr after oral administration of 20 mg ketanserin or placebo, given in a randomized, double masked, cross-over fashion. The alternative treatment was given a week later. In all patients, ketanserin significantly lowered IOP and SBP, while no variations in pupil diameter, DBP and HR were found. Moreover, after drug administration, total outflow facility, measured by conventional tonography, increased significantly. These findings indicate that oral ketanserin could represent a new antiglaucomatous drug.