Neurosteroids are a subclass of steroids that can be synthesized in the central nervous system independently from peripheral sources. Clinical studies in humans have associated these hormones with depression and postpartum mood disorders. In rodents, allopregnanolone (AlloP) has been shown to have anxiolytic and rewarding properties. These observations suggest that neurosteroids could interact with mood and motivation. However, the possible neural substrates of these effects remain unknown. In this report, we have studied the action of AlloP on the activity of the mesencephalic dopaminergic (DA) projection to the nucleus accumbens, which is considered one of the biological substrates of motivation and reward. This study was conducted by measuring extracellular concentrations of dopamine (DA) in the nucleus accumbens by means of microdialysis in freely moving rats. We studied both the direct effect of AlloP and the influence of this hormone on the DA response to an injection of morphine. AlloP dose-dependently increased the release of DA in the nucleus accumbens. Furthermore, this hormone doubled the DA response to morphine. These effects were observed for AlloP doses of 50 and 100 pmol injected intracerebroventricularly. These results suggest that the stimulatory effect of AlloP on DA could mediate some of the behavioural effects of neurosteroids and, in particular, the interaction of these hormones with mood and motivation.