Dexmedetomidine (DEX) is a highly selective alpha(2)-adrenoceptor agonist in both the central and peripheral nervous systems. Its cardiorespiratory effects have been described; however, these effects have not been reported when it is used in combination with volatile anaesthetics in rats. The cardiovascular and respiratory actions of a continuous intravenous infusion of 0.25 microg/kg/min of DEX administered to rats anaesthetized at 1 minimum alveolar concentration (MAC) of either halothane (HAL) or isoflurane (ISO) were studied. Twenty-eight rats were grouped into four treatment groups: HAL alone, ISO alone, DEX + HAL and DEX + ISO. The MAC(HAL) or MAC(ISO) was determined in each rat from alveolar gas samples at the time of tail clamping. Control MAC values, expressed as mean +/- standard deviation, were 1.31 +/- 0.1% for HAL and 1.46 +/- 0.05% for ISO. DEX reduced HAL MAC from 1.31 +/- 0.1% to 0.36 +/- 0.22% (72 +/- 17% MAC reduction) and ISO MAC from 1.46 +/- 0.05% to 0.83 +/- 0.2% (43 +/- 14% MAC reduction). Heart rate (HR) was decreased in both DEX groups at 1 MAC, with no differences between HAL and ISO. The mean arterial pressure was significantly depressed in the DEX + ISO group compared with the ISO only group. This difference in mean arterial blood pressure (MABP) was not seen between the DEX + HAL and HAL only groups. Respiratory depression was minor at 1 MAC with both inhalant anaesthetics. DEX reduced the MAC of HAL to a degree greater than it decreased the MAC of ISO. The effects of DEX on HR and ventilation were similar in rats anaesthetized with HAL or ISO at 1 MAC; however, hypotension was more pronounced when DEX was combined with ISO at 1 MAC.