A method for therapeutically treating an animal, including a human, for psychosomatic, depressive and neuropsychiatric diseases, such as anxiety, depression, insomnia, schizophrenia, epilepsy, spasm and chronic pain. Administration of a suitable DP IV inhibitor causes the reduction of activity in the enzyme dipeptidyl peptidase (DP IV or CD 26) or of DP IV-like enzyme activity in the brain of mammals and leads as a causal consequence to a reduced degradation of the neuropeptide Y (NPY) and similar substrates by DP IV and DP IV-like enzymes. Such treatment will result in a reduction or delay in the decrease of the concentration of functionally active neuronal NPY (1-36). As a consequence of the resulting enhanced stability of the endogenous NPY (1-36) caused by the inhibition of DP IV activity, NPY activity is prolonged thereby resulting among other things in functionally active NPY Y1 receptor activity thereby facilitating antidepressive, anxiolytic, analgesic, antihypertension and other neurological effects.