Fenoldopam mesylate stimulates adenyl cyclase in porcine ocular trabecular meshwork and raises intraocular pressure in humans. To clarify whether this results from direct activation of the dopamine-1 receptor or indirectly from baroreflex sympathetic stimulation after blood pressure reduction, intraocular pressure was measured in 14 patients with accelerated/malignant hypertension, randomized between intravenous fenoldopam or sodium nitroprusside. Intraocular pressure was measured with a Perkins tonometer, before and at the twentieth minute of each dose increment. In seven patients with a mean blood pressure of 232/131 mm Hg treated with fenoldopam, intraocular pressure increased in a dose-dependent fashion, from 16 +/- 1 to 20 +/- 2 mm Hg (p less than 0.005). In contrast, seven patients with a mean blood pressure of 225/134 mm Hg treated with sodium nitroprusside exhibited no change in intraocular pressure (15 +/- 1 versus 14 +/- 1 mm Hg) despite similar blood pressure reduction. Increases in heart rate were not significantly different. Rates of urinary excretion of norepinephrine plus epinephrine increased significantly relative to baseline (p less than 0.05) but were not different between groups. These data suggest that the increase in intraocular pressure with fenoldopam results from specific activation of the dopamine-1 receptor and is not caused by baroreflex sympathetic stimulation. Because dopamine-1 receptors may modulate intraocular pressure, dopamine-1 receptor blockers might be useful therapy for glaucoma.