Thirty-two ocular hypertensive subjects were treated for 90 days with either 0.5% timolol maleate or 2% epinephrine hydrochloride twice daily to one eye and both drugs to the fellow eye. The ocular hypotensive effects of timolol and epinephrine were partially additive throughout the course of this study. On the 91st day, eyes treated with epinephrine had a 25.8% mean reduction of intraocular pressure from baseline, whereas their fellow eyes treated with epinephrine and timolol had a 33.9% reduction. Eyes treated with timolol had a 27.2% mean reduction of IOP, whereas fellow eyes treated with timolol and epinephrine had a 29% reduction. These results suggest that the majority of patients being treated with either drug are unlikely to have a substantial long-term reduction in IOP when the other drug is added to their therapeutic regimen.