Basal plasma vasopressin-neurophysin (hN<sub>p</sub>I) was estimated in 50 drug-free neuropsychiatric patients classified according to the Research Diagnostic Criteria. The hN<sub>p</sub>I concentration was higher in the 5 manics (0.76 ± 0.15 ng/ml) than in the 16 schizophrenics (0.53 ± 0.08), 12 minor depressed (0.54 ± 0.06) and 17 major depressed (0.48 ± 0.10; p < 0.05). Thus, those results confirm our initial observation of an increased vasopressinergic function in the manic compared to the depressed phase in one bipolar patient. Whether this increase in the vasopressin release is a consequence of the neuropsychiatric disorders or initiates and/or participates in their pathophysiology remains to be elucidated. The hypothetic consequence on water metabolism of such an increase remains also to be defined.