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The forced swim test (FST) is a widely used animal model of depression and antidepressant drug screen. Rats are forced to swim on two test days in a restricted space from which there is no escape. On the first test day the rats attempt to escape and then become largely immobile; on the second test day the onset of immobility is more rapid. Immobility is said to reflect a state of lowered mood or "behavioral despair", but the validity of the FST as a model of depression has been questioned. We show here that whatever psychological states the FST may induce, immobility is water temperature dependent and thermoregulatory. In Experiment 1, separate groups of rats were first tested in water of 15, 20, 22, 25, 30, 35, 37, or 40° C. When retested at the same temperature, reduced activity was evident only in those groups tested above 20° C and below 37° C. On a third test, rats previously tested in 35^O C water failed to show reduced activity in 15° C water, whereas rats previously tested at 15° C water did exhibit reduced activity when tested in 35° C water. Thus, activity was dependent on current water temperature rather than prior experience. In Experiment 2, activity and body temperature were monitored during 30 min swim tests in 27° C water. The more the animals moved, the greater the loss of body temperature. The results are consistent with a hypothesis that immobility in the FST is an adaptive thermoregulatory response that increases survival by minimizing convective heat loss. This interpretation is also aligned with best practices for survival of humans in water that is below thermoneutral.