When rats or mice are forced to swim in a deep cylinder with tepid water they become nearly immobile and cease trying to escape. Immobility is reduced when rats and mice are treated with a wide variety of antidepressants. Immobility in a swimming tank was initially conceived of as “behavioural despair” by Porsolt. Other views were that immobility is an attempt at conserving energy or an evolutionary preserved coping strategy where immobility represent the psychological concept of “entrapment” described in clinical populations. Rodent models of depression are mainly based on predictive validity, objectivity of measured response and their high reliability and reproducibility.