A randomised, double-masked clinical study was conducted in patients with primary open-angle glaucoma to determine if timolol and epinephrine have an additive effect in lowering intraocular pressure. Sixteen patients were randomly assigned to one of 2 treatment sequences (timolol alone, supplemented after 2 weeks with epinephrine, and vice versa). An initial additive effect in lowering intraocular pressure was found in both sequences. However, after several weeks of combined therapy complete loss of additive effect was found. Patients who were treated first with epinephrine for 2 weeks and then supplemented with timolol had significantly lower intraocular pressures for at least 2 weeks than patients in the reverse treatment sequence. Epinephrine treatment alone caused a significant increase in facility of outflow, but this effect did not occur with simultaneous timolol treatment. The results are discussed in terms of possible fundamental beta and alpha adrenergic influences on aqueous dynamics and their potential clinical relevance.