Intravenous injection into rabbits and cats of an aqueous extract of hornet venom sac, produced a significant reduction in the intraocular pressure. The mean reduction of the ocular pressure was between 7 and 15 mm Hg within one hour past injection and it lasted 6-12 h or in several instances even more than 24 h. The effect is produced by some thermolabile high-molecular-weight fraction(s), is dose-dependent and is reproducible by repetitive injections. Side effects may include diarrhea, micturition and lethargy, but usually there is no marked effect on the size of the pupil.