Pharmacological data indicate that prodynorphin peptides and exogenous kappa agonists affect opioid tolerance and dependence. In order to elucidate the activity of the endogenous prodynorphin system during opiate tolerance and dependence, we investigated the effect of single and repeated morphine administration on the alpha-neoendorphin tissue level, its in vitro release, and the prodynorphin messenger RNA level in the nucleus accumbens and striatum of the rat. Acute and repeated morphine administration (14 days, increasing doses, 20-100 mg/kg, i.p.) increased the level of alpha-neoendorphin in the nucleus accumbens after 3 h; a similar effect was observed at 24 and 48 h after the last chronic morphine injection. On the other hand, the basal and stimulated (K+, 57 mM) release of alpha-neoendorphin from nucleus accumbens slices were significantly elevated only at 24 h after the last morphine injection. The prodynorphin messenger RNA hybridization signal in the nucleus accumbens was enhanced at 3 h after acute morphine injection, whereas repeated morphine administration decreased the messenger RNA level at that time point. Upon late chronic morphine withdrawal (at 24 and 48 h), the prodynorphin messenger RNA level in that tissue was significantly elevated. In the striatum, single morphine administration had no effect on the alpha-neoendorphin tissue level, release of the peptide, and prodynorphin messenger RNA level. On the other hand, chronic injection of morphine elevated all those parameters. The tissue level of alpha-neoendorphin was elevated at 3 h, and was back to normal at 24 and 48 h after the last drug injection. Both the basal and stimulated alpha-neoendorphin release from striatal slices was significantly increased at all the time points studied. Repeated morphine administration elevated the striatal prodynorphin messenger RNA level at 24 and 48 h after the drug withdrawal. Addition of morphine to the incubation medium reduced the basal release of alpha-neoendorphin in both the nucleus accumbens and striatal slices in naive animals, whereas the stimulated release was attenuated in the latter tissue only. The present study indicates that withdrawal of chronic morphine leads to enhancement of the prodynorphin neurons activity in the nucleus accumbens and striatum of the rat. It is suggested that these effects may participate in the mechanism of aversive reactions during withdrawal.