The effect of desmethylimipramine (DMI) on the hypothalamic and neurohypophysial vasopressin content was investigated in normal and pinealectomized male rats. The vasopressor activity of the hypothalamus and neurohypophysis was bioassayed by the method of Dekański (1952). A single i.c.v. injection of DMI (20 micrograms; dissolved in 10 microliters of normal saline) decreased vasopressin content in the neurohypophysis of non-operated and sham-operated animals. In pinealectomized rats the vasopressin content in the neurohypophysis diminished and was not modified by an i.c.v. DMI injection. Repeated treatment with DMI (i.p.; 10 mg/kg, twice a day over five days) resulted in a distinct increase of the hypothalamic and neurohypophysial vasopressin in both normal and pinealectomized rats.