Concomitant administration of the alpha 2-adrenoceptor agonist, clonidine, and the alpha 1- and non-selective beta-adrenoceptor antagonist, labetalol, was studied in 12 hypertensive outpatients treated with thiazide diuretics. Clonidine, 0.15 mg b.i.d., effectively lowered supine, standing and isometric blood pressure values when administered to patients on cyclothiazide, 2.5 mg q.d. Addition of labetalol, 200 mg b.i.d., to the diuretic-clonidine treatment resulted in further reduction of blood pressure at rest both in the supine and standing position. In contrast, during isometric work, addition of labetalol to the diuretic-clonidine regimen did not exhibit the same additive antihypertensive effect. When clonidine was given in a dose of 0.3 mg once daily in the evening and the other two drugs as previously, the same antihypertensive effect was observed in the afternoon. Labetalol did not provoke new side-effects of its own.