Clinical studies were conducted to determine the relationship between simultaneous changes in heart rate, blood pressure, and intraocular pressure in systemic normotensive (N=8) and hypertensive (N=8) open-angle glaucoma patients (N=16) after inhalation of tetrahydrocannabinol (THC). Insignificant changes occurred in arterial and ocular pressures after placebo pertubations. The functional responses after 2.8% THC inhalation in sitting normotensive and hypertensive patients included invariable increases in heart rate (range 20-62 beats/min greater than control) followed by substantial decreases in systolic pressure (range 12-52 mmHg less than control), diastolic pressure (range 6-28 mmHg less than control), and intraocular pressure (range 6-21 mmHg less than control). The intensity and duration (3-4 hours) of the arterial and ocular pressure responses to THC were greater in hypertensives than in normotensive patients. The salient observation after THC inhalation was that the changes in ocular pressure paralleled the changes in blood pressure in each glaucoma patient. These findings suggest that the positive chronotropic response to THC tends to maintain cardiac output which limits further decreases in blood pressure and the capillary filtration of aqueous humor decreases or the reabsorption of aqueous humor increases because of the systemic hypotensive effect attending THC inhalation.