Recent clinical trials show that serotonergic psychedelics, including the prototypical hallucinogen lysergic acid diethylamide (LSD), possess a great promise for treating affective disorders. Interestingly, LSD displays strong functional activity on 5-HT_2B receptors and a modulatory role of the latter receptors in anxious and depressive-like behaviors has been reported. Using behavioral and in vivo electrophysiological tools in naive rats, the effects of acute administration of LSD were evaluated in the: forced swim test (FST), open field test, foot shock-induced ultrasonic vocalization, on the head-twitch response (HTR) and on the dorsal raphe serotonin 5-HT cell activity. By comparison, the antidepressant-, anxiolytic- and hallucinogenic-like effects of LSD were then assessed in naïve mice using the FST, the black & white box test and HTR. We show here that acute administration of LSD induced fast antidepressant-, anxiolytic- and hallucinatory-like effects as well as a suppression of 5-HT neuronal activity that were all counteracted by the selective pharmacological blockade of 5-HT_2B receptors, including the potent and selective 5-HT_2B receptors antagonist RS-127445. Interestingly, depletion of 5-HT prevented the action of LSD in FST and HTR. In contrast in mice, acute injection of LSD failed to produce an antidepressant- or anxiolytic-like response, and the hallucinogenic-like effect of LSD was not altered by a pretreatment with RS-127445. Together, these findings indicate that LSD, acutely administered, acts as a rapid-onset antidepressant in naïve rat, but not in naïve mice, through mechanisms involving activation of 5-HT_2B receptors.