The response to oral clonidine hydrochloride loading in 36 severely hypertensive patients is presented. Each patient initially received 0.2 mg of clonidine hydrochloride, followed by 0.1 mg each hour until a dose of 0.7 mg had been given, or the diastolic blood pressure (BP) reached a predetermined goal (110 mm Hg or total fall of at least 20 mm Hg). Only two patients (6%) failed to reach this goal. Supine BP in the group fell from 212 +/- 22 (SD)/139 +/- 11 (SD) mm Hg to 151 +/- 21 (SD) mm Hg at six hours. The average dose of clonidine required was 0.45 mg and control was maximized at five hours. The response to oral clonidine loading in the individual patient was not predictive of the eventual dose of clonidine necessary to achieve acceptable BP control at two weeks. Oral clonidine loading is safe and effective in the management of "hypertensive urgencies" and offers several advantages over parenteral antihypertensive agents in this clinical situation.