We have examined and compared the effect of systemic CI988, an antagonist of the cholecystokinin B receptor, and morphine on the flexor reflex in rats with intact sciatic nerves and after unilateral axotomy. Intravenous injection of 1 mg kg-1 morphine caused a moderate depression of the flexor reflex in normal and axotomized rats. The duration of morphine-induced depression was significantly shortened in axotomized animals. In contrast, the brief depressive effect of intravenous CI988 on the flexor reflex was significantly enhanced in axotomized rats. Combination of morphine and CI988 resulted in a significant potentiation of reflex depression in rats with intact or sectioned sciatic nerves. It is concluded that blockade of CCK-B receptor potentiated morphine-induced antinociception in rats after peripheral nerve injury.