A comparative tonographic study was performed in 10 primary open angle glaucoma patients (15 eyes) and 7 patients with suspected glaucoma (14 eyes) during a 7-months' period. The patients were treated twice daily with guanethidine 3% and adrenaline 0.5% (GA). The fall in intraocular pressure (IOP) of 44% in primary open angle glucoma patients was mainly due to an inhibition of aqueous humour production (54%; p < 0.005) and to a lesser extent an increase in outflow facility. In the suspected glaucoma patients, the fall in IOP of 43% was due to an inhibition of the aqueous rate (46%; p < 0.005) and to an increase in outflow facility (64%; p < 0.005). The increase in outflow facility during treatment was significantly different between both groups of patients. It suggests degeneration of the receptors mediating outflow mechanism in the patients with open angle glaucoma. For both groups of patients it is shown that during treatment with GA there is not only a supersensitivity for adrenaline 0.5% of the mechanisms that mediate the inhibition of the aqueous rate, but also of the mechanisms that mediate outflow facility.