Several recent studies have demonstrated that alprazolam and lorazepam, administered at low doses to healthy volunteers, improve cognitive functions and psychomotor performances. Paradoxical effects of low-dose benzodiazepines have been also observed in mice, in experimental pharmacology. The aim of this work was to determine, in rat, the effect of similar low-doses of benzodiazepines on spontaneous locomotor activity and performance in the elevated zero-maze, and to investigate the underlying neurobiological mechanisms. The dose-effect and the time-course of the action were studied for both compounds. Spontaneous locomotor activity was measured using a photoelectric actimeter. The level of anxiety of the animals was assessed in the elevated zero-maze. Dopamine, serotonin, and their metabolites were assayed in the extracellular striatal fluid of the awake rat, obtained by microdialysis, by HPLC--EC. Spontaneous locomotor activity observed in rats given low-dose alprazolam and lorazepam evidenced a stimulatory effect only with alprazolam. The effect was maximum 90 min after administration of 0.0050 mg/kg alprazolam. An anxiogenic-like action was evidenced with the elevated zero-maze for the two compounds. We observed a statistically significant increase in striatal dopamine concentrations only with alprazolam, during the period corresponding to the behavioral stimulatory effects. We also showed a marked trend towards increased levels of serotonin with alprazolam but this modification was not significant, in spite of statistically significant variations of 5-HIAA. In the rat, behavioral stimulatory effects of low-dose benzodiazepines is evidenced with alprazolam but not lorazepam. This effect could be explained, at least in part, by increased extracellular dopamine concentrations in the striatum. Their different structures could explain the different pattern observed for the two benzodiazepines.