The forced swim test measures the time spent swimming versus the time spent floating in a tall cylinder filled with water. Rats or mice will swim in the water looking for an escape route. After some time the animal may stop swimming and begin floating. This is regarded as having "given up the search."
Treatment with desipramine, a tricyclic antidepressant, decreases passivity and increases escape-related climbing activity. Treatment with fluoxetine, a selective serotonin reuptake inhibitor (SSRI) antidepressant, decreases passivity and increases swimming time.