• We investigated the effects of the topical application of acepromazine maleate on the intraocular pressure (IOP) in 27 adult rhesus monkeys. The monkeys were divided into two groups: group 1 (16 monkeys) had both eyes normal, and group 2 (11 monkeys) had experimental chronic glaucoma in one eye and a normal fellow eye. One drop of 1% acepromazine maleate solution was instilled in one eye of monkeys in group 1 and in the glaucomatous eye of monkeys in group 2; the other eye served as the control. The IOP was measured before drug administration and 1, 4, 8, 24, and 32 hours after, with detailed slit-lamp examination of the anterior segment. Acepromazine produced no change in IOP in eyes in group 1, but it produced a fall in pressure in all eyes with high IOP in group 2, evident 1 hour after instillation, maximal between 4 and 8 hours, and still remaining after 32 hours. The pupil showed no change in size, but a transient ptosis was observed in the treated eye in all monkeys.