Ketanserin has been characterized as a relatively selective antagonist for serotonin (5-HT2) receptors. However, recent evidence suggests that ketanserin can also antagonize activity at alpha 1-adrenoceptors. Topically applied ketanserin lowered intraocular pressure (IOP) in rabbits, cats, and monkeys. Ketanserin also suppressed ocular hypertension induced by waterloading and the IOP recovery rate in response to hypertonic saline. In contrast, the ocular hypotensive activity of ketanserin was markedly attenuated in sympathectomized (SX) rabbits. In the cat nictitating membrane, ketanserin produced dose-related inhibition of contractions elicited by neuronal stimulation and by intra-arterial injection of norepinephrine. These data demonstrate that ketanserin: 1) lowers IOP more effectively in eyes with intact sympathetic innervation than in SX eyes, 2) appears to lower IOP by suppressing aqueous inflow, and 3) acts primarily by antagonizing the action of norepinephrine on alpha 1-adrenoceptors.