Beta-adrenergic antagonists are generally considered to lower intraocular pressure (IOP) of both eyes after unilateral instillation. In order to determine whether timolol also lowers IOP in the contralateral eye and to what extent, pressure curves were established in 14 normal, young subjects. Eye pressure curves on both eyes of each subject were measured before and 1 week after timolol 0.5% instillation twice daily in one eye. All subjects had an IOP decrease in the treated eye, but no subjects had a statistically significant IOP decrease in the contralateral eye. The mean IOP reduction was 26% in the treated eye, and only 3% in the contralateral eye. These results suggest that, in most cases, timolol does not lower IOP in the contralateral eye after unilateral instillation in normal subjects in contrast to certain glaucoma patients. These results suggest two different actions for timolol: (1) a local action in the treated eye, and (2) a systemic action where the pressure-lowering effect in the untreated eye is significant only in some pathologic conditions.