Approximately 90% of arginine vasopressin circulating in blood is associated with the platelets. Studies in six patients with cranial diabetes insipidus indicate that the platelet vasopressin originates in the hypothalamus and is taken up from plasma. Extracted platelet vasopressin is chromatographically identical to the posterior pituitary peptide and biologically active. Infusion of low-doses of exogenous vasopressin into 10 normal male subjects caused a marked rise in both plasma and platelet hormone concentrations. Stimulation of endogenous vasopressin release by hypertonic saline (0.51 mol/l) infusion for 135 min in five male subjects also caused a rise in platelet and plasma peptide concentrations. In seven male smokers, the deep inhalation of two consecutively smoked unfiltered cigarettes caused a significant rise in both platelet and plasma vasopressin. Baseline platelet vasopressin concentrations in the smokers were higher than in the non-smoking males even after 10 hours abstinence.