Opioid peptides, through μ and δ receptors, play an important part in reward. In contrast, the role of κ receptors is more controversial. We examined the possible positive reinforcing effects of a selective κ agonist, RU 51599, by studying intravenous self-administration in the rat. The effect of RU 51599 on dopamine release in the nucleus accumbens was also studied, as opioids and dopamine seem to interact in the mediation of reward. The behavioural and dopaminergic effects of RU 51599 were compared with those of the μ agonist heroin. Rats self-administered both RU 51599 (6.5, 20 and 60 μg/inj) and heroin (30 μg/inj) at low ratio requirement. When the ratio requirement, i.e. the number of responses necessary to receive one drug infusion, was increased, self-administration of RU 51599 rapidly extinguished, whereas self-administration of heroin was maintained. Intravenous infusion of RU 51599 (100, 200 and 400 μg) dose-dependently decreased (25, 30 and 40%, respectively) extracellular concentrations of dopamine, as measured by means of microdialysis in freely moving rats. In contrast, heroin increased accumbens dopamine (130% over baseline). These results indicate that κ receptors, similarly to μ ones, can mediate positive reinforcing effects of opioid peptides. However, the strength of the reinforcement is very low for κ receptors. This suggests that changes in accumbens dopamine do not correlate with the capacity of a stimulus to induce reward or aversion. In contrast, a parallel seems to exist between an increase in accumbens dopamine and the drive to reach or obtain a positive reinforcer.