In a 1-day, 1-dose, double-masked, randomised trial, with each of 12 patients acting as his/her own control, atenolol drops 4% (a selective beta1-adrenergic blocker) produced a significantly greater fall in ocular tension measured by applanation than did adrenaline drops 1% (P is less than 0.01 Wilcoxon matched pairs signed ranks test). The mean differences, which favoured atenolol, between the falls in pressure produced by these 2 drugs at 1.5 hours, 3.5 hours, 5.5 hours, and 7 hours after instillation of the drops was 2.1, 4.6, 4.0, and 3.6 mmHg, respectively. Long-term studies would be required before any conclusion was justified about the relative merits of these 2 drugs in the treatment of glaucoma. There was no significant difference between the ocular hypotensive effects of atenolol-then-adrenaline and adrenaline-then-atenolol. It was disappointing that the expected adjuvant effect of atenolol's preceding adrenaline was not found-rather the reverse. Atenolol alone, however, was significantly better than atenolol-then-adrenaline (P is less than 0.02 Wilcoxon matched pairs signed ranks test), and there was also some indication that it was superior to adrenaline-then-atenolol. The response to adrenaline did not differ markedly from the response to the combination in either order.