A single session of uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stress is known to selectively potentiate subsequent morphine-conditioned place preference in a dorsal raphe nucleus (DRN) serotonin (5-HT) dependent manner. Here, in vivo microdialysis is used to test the hypothesis that prior IS, but not ES, will potentiate morphine-induced dopamine (DA) efflux in the nucleus accumbens (NAc) shell and that this will occur by a pathway involving DRN 5-HT neurons. Male Sprague-Dawley rats were exposed to yoked IS, ES, or no stress. Twenty-four hours later, morphine (3 mg/kg s.c.) or saline was administered during microdialysis. As predicted, prior IS selectively potentiated morphine-induced DA, but not 5-HT, efflux in the NAc. This potentiation was due to morphine's action in the DRN because it was blocked by intra-DRN microinjection of the opioid antagonist naltrexone (10 microg). IS potentiation of morphine-induced DA efflux in the NAc was also dependent upon activation of 5-HT neurons in the DRN because it was blocked by intra-DRN microinjection of the 5-HT1A autoreceptor agonist 8-hydroxy-2-di-n-(propylamino)-tetralin (1 microg). No effect of IS was found on morphine-induced 5-HT or DA efflux in the ventral tegmental area. These results suggest a neural substrate for stress potentiation of morphine reward involving 5-HT neurotransmission in the DRN.