Depression is characterized by a lack of "motivation" rather than a lack of "physical space" to move around. This study was designed to evaluate predictivity of an open space swimming test for antidepressant activity of various antidepressants in rats. Without drug treatment, rats showed a significant reduction in the distance moved (increased immobility) over successive trials in an open space water pool. Three major classes of antidepressants and a selective serotonin reuptake inhibitor (SSRI) were tested. Repeated treatment (10 mg/kg x 3 per day) of imipramine, a prototypical tricyclic antidepressant, iproniazid, a monoamine oxidase inhibitor, mianserin, an atypical antidepressant, and alaproclate, an SSRI, all significantly reduced the immobility. These results suggest that the open space swimming test is highly predictive of antidepressant action and is more sensitive to the drug treatments. The measurement is more objective than that of the forced swimming test and does not involve judging and scoring the animals' movement or lack of movement by investigators. The demonstrated effectiveness of three major types of antidepressants and an SSRI suggests that the effects on the test are not restricted to a particular underlying molecular mechanism of action. Thus, this swimming test shows promising potential as a screen for novel antidepressants and, perhaps, for revealing some of the underlying pathophysiology of depression.