Comparisons between tonography results confirm an almost identical action of timolol and diamox: With both, the intraocular pressure and the minute volume of aqueous flow are significantly reduced, while the resistance to the aqueous outflow remains unchanged. In a consecutive tonometric study it was proved that timolol, when fully developed, inhibits the pressure-lowering effect of diamox (500 mg i.v.) when applied two hours before the latter. Thus the action of timolol is not additive but rather substitutive to the action of diamox. Its maximal pressure-lowering effect corresponds to the half of the aqueous outflow-pressure gradient.