Autonomic neuropathy may contribute to the pathophysiology of both open-angle and normal-pressure glaucoma. However, autonomic function has not been studied extensively in these diseases. We evaluated baroreflex control of the heart and blood vessels in open-angle and normal-pressure glaucoma. We studied 14 patients with open-angle glaucoma, 15 with normal-pressure glaucoma and 17 controls. Sinusoidal neck suction (0 to -30 mmHg) was applied at 0.1 Hz to assess the sympathetic modulation of the heart and blood vessels, and at 0.2 Hz to assess the effect of parasympathetic stimulation on the heart. Baseline recordings showed no significant differences between the groups. The RR-interval response of the controls to neck suction at 0.1 Hz (3.88+/-0.32 to 6.65+/-0.44 lnms2) was significantly greater than that of the open-angle glaucoma patients (4.22+/-0.28 to 5.56+/-0.26 lnms2) and the normal-pressure glaucoma patients (4.53+/-0.27 to 5.53+/-0.37 lnms2) (P<0.05). The low-frequency power of diastolic blood pressure increased significantly in the controls (0.48+/-0.08 to 2.76+/-0.72 mmHg2; P<0.01) during 0.1 Hz neck suction, but did not change significantly in patients with either open-angle glaucoma or normal-pressure glaucoma. The RR-interval response in the control group (3.45+/-0.52 lnms2) to neck suction at 0.2 Hz was significantly greater than that of the normal-pressure glaucoma patients (1.84+/-0.32 lnms(2)) and the open-angle glaucoma patients (1.58+/-0.35 lnms2) (P<0.05). The decreased sympathetic and parasympathetic modulation during baroreceptor stimulation in patients with open-angle glaucoma and normal-pressure glaucoma suggests that autonomic dysfunction may contribute to the pathogenesis of both diseases.