Cardiopulmonary effects of medetomidine (20 micrograms/kg)-midazolam (0.3 mg) (Me-Mi) were compared with those of medetomidine alone (80 micrograms/kg) (Me80) in dogs. The intramuscular administration of this combination caused bradycardia and transient mild pressor response. Heart rate decreased soon after the administration and remained significantly below the baseline value with average values of 50-70 beats/min. Blood pressure increased to its maximum within 5 to 10 min then decreased gradually. Cardiac index decreased corresponding the decrease in heart rate. However these changes were less profound than those of Me80 indicating significantly higher values in cardiac index and lower values in systemic vascular resistance. Effects on the respiratory function were slight. The reduction of the dose of medetomidine to one-fourth in Me-Mi was effective to reduce the adverse effect of medetomidine, especially in peripheral vasoconstriction. Atipamezole effectively reversed cardiopulmonary effects induced by medetomidine-midazolam. Heart rate and cardiac index increased and systemic vascular resistance decreased significantly after administration of atipamezole. The possible use of an antagonist as a reversal agent might enhance the value and availability of medtomidine-midazolam as a chemical restraint agent in dogs.