Behavioral immobility during forced swim is usually considered a consequence of inescapable stress, and is used to screen antidepressant drugs. However, immobility in this test may also result from inhibition of neural functions because of brain hypothermia due to body cooling. To explore this possibility, we measured brain temperature dynamics during a 10-min forced swim in cold (25 degrees C) and warm (37 degrees C) water and correlated brain temperatures with behavioral changes. Cold water forced swim resulted in significant brain hypothermia (-6-7 degrees C) and immobility, while no immobility was observed during warm water forced swim, when brain temperature transiently increased (0.5 degrees C) then decreased below baseline in the post-swim period. These data suggest that immobility, which rapidly develops during forced swim in cold water, may result from dramatic inhibition of neural functions because of severe brain hypothermia.