Long-term depressive behavior was modeled via immunization of white rats with isatin (an endogenous MAO inhibitor) covalently bound to a carrier antigen. The immunization of rats against isatin and several exogenous MAO inhibitors (pargyline and deprenyl) resulted in long-term (2 months) depressive-like behavior with elements of anxiety and an increased activity of MAO in the brain. In contrast, immunization against endogenous inductors of anxiety (cholecystokinins 4 and 3) induced prolonged antidepressant and anxiolytic effects. The authors discuss the advantages of the immunochemical approach, both to modeling of depression and pathological anxiety, and to their long-lasting correction.