Inosine, a major breakdown product of adenosine by the enzyme adenosine deaminase, binds to adenosine receptors and initiates intracellular signaling events. Adenosine and inosine have already been described as capable of modulating depression-like behavior in adult rodent model. In this study, we investigated the antidepressant-like effect of inosine, acutely administered (10 or 50 mg/kg, intraperitoneal), in postnatal day 28 male Sprague-Dawley rats. The effects of inosine were assessed using the forced swimming test and locomotor activity test. Changes in the extracellular signal-regulated kinases (ERK)-cyclic AMP response element binding protein (CREB) signaling pathway in the hippocampus and prefrontal cortex were examined by western blot analysis. The results showed that inosine had an antidepressant-like effect in the forced swimming test in adolescent rats without affecting locomotor activity. Furthermore, we found that inosine increased the activity of ERK and CREB in the hippocampus and prefrontal cortex. Inosine administered intraperitoneally may alleviate depression-like behaviors in adolescent rats, and the ERK-CREB signal system may be the target of the antidepressant action of inosine.