Antidepressant effects brought about by olfactory stimulation with various odorants were investigated with the forced swimming test, a reliable means for screening antidepressant effects. Lemon odor significantly reduced total immobility time and potentiated the imipramine-induced reduction of total immobility time in the test. This synergistic effect of lemon odor and imipramine was not due to lemon odor decreasing the metabolism of imipramine. Lemon odor decreased locomotor activity in the open field, suggesting its effects to differ from those of psychostimulants but to be similar to those of antidepressants. The effects of citral, which is one of the main components of lemon odor, were as strong as those of lemon odor. The remaining odorants tested in this study failed to have any effects on total immobility time in the forced swimming test or on locomotor activity in the open field.