- Background: Eicosapentaenoic acid (EPA) is one of the major components of fish oils. Omega-3 fatty acids, particularly EPA, have been hypothesized to play a role in the etiology, pathogenesis and treatment of mood disorders. Clinical studies have shown beneficial effects of omega-3 fatty acids in major depression, bipolar disorder and other psychiatric disorders. Objective: The present study design evaluates the effect of EPA in the Porsolt forced swimming test. Results: EPA alone did not reduce the immobility time and did not enhance the anti-immobility effect of a low dose of imipramine. Contrary to the hypothesis, EPA slightly increased the immobility time, and in some experiments tended to reduce the anti-despair effect of imipramine. Conclusion: The present results do not provide an animal model for the antidepressant effect of EPA as demonstrated in clinical experiments. The mechanism of EPA antidepressant action is unknown and the Porsolt forced swimming test could be non-sensitive for its antidepressant properties.