Tolvaptan is a vasopressin receptor antagonist. Vasopressin (a.k.a. ADH or antidiuretic hormone) helps to regulate water retention by absorbing water in the collecting ducts of the nephron.

The reabsorption of water back into the blood (at the medulla) is controlled by the anti-diuretic hormone (ADH). ADH is produced by the part of the brain that monitors water levels in the blood.