The effects of opiates on vasopressin secretion have been controversial for many years. This is probably due to the existence of different types of opioid receptors and to the lack of specificity of the compounds used. Specific kappa agonists, which have been described recently, produce a marked diuretic effect without any associated increase in electrolyte elimination. They seem to exert their effects through an interaction with kappa receptors situated on nerve terminals and/or pituicytes. These receptors could be directly coupled to L-type calcium channels, their activation leading to a decrease in the effectiveness of action potentials to evoke vasopressin secretion from nerve terminals in the neurohypophysis. This mechanism of action may explain the decrease in plasma vasopressin levels induced by kappa agonists.