The antihypertensive agent, clonidine, has a marked sedative effect. We studied whether clonidine also deepens halothane anaesthesia. Eight rabbits were anaesthetized with and without clonidine premedication in a cross-over study. Clonidine premedication (50 microgram/kg subcutaneously) was administered three times daily for 3 days. Tolerance to pain during halothane anaesthesia was tested by compressing the ear with a vessel clamp. Halothane concentrations were determined by gas chromatography. The rabbits premedicated with clonidine tolerated painful stimuli without reactions at lower halothane concentrations in arterial blood and inspired air than unpremedicated rabbits. MAC calculated from blood concentrations was 1.29% for unpremedicated and 1.09% for clonidine-premedicated rabbits. The results suggest that clonidine diminishes the anaesthetic requirement in halothane anaesthesia.