Unpredictable chronic mild stress model was developed as an animal model of depression more than 20 years ago. Essential for this model is that after prolonged exposure of tested animals to a series of unpredictable mild stressors, a condition similar to anhedonia develops, which is observed in the majority of depressive disorders. Unpredictable chronic mild stress model is used nowadays in numerous studies related to the neurobiological and biochemical changes associated with depressive illness. Their results confirm that chronic unpredictable mild stress induces in tested animals a number of changes, which reflect those seen in depressive disorders. Because the effects of unpredictable chronic mild stress can be used in a more accurate diagnosis of the pathophysiology of depressive illness and expand knowledge of its pharmacotherapy, therefore research in this area has been continued all the time.