Stress alters the sensitivity to drugs of abuse and is, therefore, considered to be an important contributory factor to drug-seeking behaviour. There is only a limited amount of information available on stress-induced alterations in the behavioural response to opioids. We thus evaluated the influences of different stressors (restraint, handling, social defeat) on the locomotor effects induced by morphine. Further the importance of additional factors such as the number of stress events or the delay between stress and locomotor testing on stress-induced alterations were evaluated. Since these modulatory effects of stress on the locomotor effects of morphine might be mediated via the release of endogenous corticosteroids we also tested the influence of repeated intermittent and chronic administration of corticosterone (CORT) and the synthetic corticosteroid dexamethasone (DEX) on the locomotor response to morphine. Enhanced morphine-induced locomotor activity was observed in response to the repeated application (three times) of all stressors: restraint, handling and social defeat. An augmentation of the locomotor effects of a low (1 and 5 mg/kg) but not of a high dose (10 mg/kg) of morphine was seen after three, but not after one stress event. In addition, the repeated application of restraint stress (three times) resulted in an augmentation of morphine-induced locomotor stimulation 3 days, but not 1 or 10 days , after the last stress event. Similarly the repeated intermittent and chronic administration of corticosteroids, in particular of DEX, increased morphine's efficacy in stimulating locomotor activity. Our results show that stress is able to alter the locomotor stimulant effects of morphine in rats--a phenomenon called stress-induced behavioural sensitization. Moreover, these stress-induced alterations depend upon temporal factors such as number of stress events and the interval between stress and locomotor testing. Further, stress-induced CORT-release seems to be involved in stress-induced behavioural sensitization to morphine.