Timolol ophthalmic solutions 0.5%, 1.0% and 1.5% lowered intraocular pressures significantly in normal human volunteers. Maximum lowering of the intraocular pressures was reached at 2 h with the 0.5% solution of timolol and at 1 h with the 1.0% and 1.5% timolol ophthalmic solutions. The effect lasted the full 7 h of observations. No objective or subjective evidence of ocular irritation could be attributed to the drug. A single dose of timolol applied topically to the eyes of normal human volunteers had no effect on pupillary size, visual acuity, blood pressure or pulse rate.