Rodents forced to swim in a narrow space from which there is no escape will, after an initial period of vigorous activity, adopt a characteristic immobile posture, making only those movements necessary to keep their heads above the water. It was hypothesized that immobility reflected the animals' having learned that escape was impossible and their having given up hope. Immobility was therefore given the name "behavioral despair". Immobility was subsequently found to be reduced by a wide range of clinically active antidepressant drugs. This simple behavioral procedure has since become a useful test for screening novel antidepressants in rats and is presented in this unit. An equivalent procedure in the mouse is also described along with a "dry" version of the test where immobility is induced simply by suspending the mouse by the tail.