The authors studied the ocular haemodynamics by means of the Color Doppler Imaging (CDI) technique and the levels of cyclic guanosine monophosphate (cGMP), the intracellular mediator of NO action in plasma and the aqueous humour in a Normal Pressure Glaucoma group and in a normal group. They found significant alterations of both the velocities, systolic and diastolic, in the Ophthalmic Artery and lower cGMP levels in NPGs than in the controls. These data suggest that a disorder of NO regulation processes might be involved in blood supply to the optic nerve and in aqueous humour outflow.