The effects of acute and chronic fluoxetine treatment in intact and anxiety-depressive male and female inbred mice of the C57BL/6J strain were studied. The gender differences in the behaviour of mice in the tests estimating anxiety, locomotion and exploration activity, communication, and depressive-like state after fluoxetine injection were established. The dependence of fluoxetine treatment on normal or pathological state in mice was discovered. It was concluded that use of the animals in pathological condition and chronic (but nor acute) fluoxetine treatment are represented as the most correct estimative means of antidepressant efficiency.