Vasopressin is a hormone secreted by the pituitary gland. In the human body, vasopressin serves both as a neurotransmitter and as a regulatory hormone for the kidneys.

When a person is water deprived, vasopressin secretion will be stimulated, and the kidneys will excrete less water, reducing the deprivation by enabling the body to use water that would have been lost by urination.

In the brain, vasopressin seems to play an important role in male sexual behavior. When male rats were given a subtance that prevented vasopressin from working, they no longer performed sexual behaviors. Also, castration is associated with both a decline in sexual performance and a decrease in vasopressin production. When drugs are administered to enhance vasopressin production, sexual behavior returns.