The interfering effect of sulphinpyrazone, a uricosuric agent which reduces the activity of cyclo-oxygenase, with the antihypertensive activity of oxprenolol, a non-cardioselective beta-blocker with sympathomimetic activity, has been evaluated. Ten patients with primary arterial hypertension of mild to moderate degree entered a randomized doubleblind cross-over study versus placebo. They were given oxprenolol + placebo or oxprenolol + sulphinpyrazone for 15 days, and then the treatments were crossed-over for a further 15 days. Oxprenolol significantly reduced blood pressure (161±3/101±1 vs 149±4/96±2 mmHg) and heart rate (72±3 vs 66±3 beats/min). During administration of the combination with sulphinpyrazone the blood pressure increased to its pretreatment level (156±5/101±2 mmHg). The effect of oxprenolol on heart rate was not influenced by the combined treatment (67±6 beats/min). The results may be explained by 1) sulphinpyrazone-induced inhibition of prostaglandin synthesis, which could interfere with the antihypertensive activity of oxprenolol, or 2) sulphinpyrazone-induced acceleration of the metabolism of oxprenolol. © 1986 Springer-Verlag.