We studied the relationship of blood and brain levels of imipramine (IMI) and/or its active metabolite desipramine (DMI) to behavioural response, in the tail suspension test (TST). Compared to the control group, doses of 3.75, 7.5, 15, 30 and 60 mg/kg of IMI given intraperitoneally reduced immobility scores significantly. There were significant negative correlations between doses, brain levels of IMI, DMI, IMI + DMI, blood levels of IMI, DMI, IMI + DMI and the animal behaviour. Higher level of IMI and its metabolite in brain and in blood did not impair anti-immobility effect. These findings agree with most results found in men treated with IMI, and suggest a linear or sigmoid relationship between the antidepressant concentrations and its biological response.