Bulimia nervosa is a psychiatric syndrome associated with intense hunger, deficient satiety mechanisms, an obsessional preoccupation with the adverse consequences of eating, ritualistic binge eating, and subsequent purging to forestall the effects of the binge. The morbidity of this illness reflects both the psychological suffering associated with a life organized around pathological eating behaviors, as well as medical complications such as fluid and electrolyte imbalances that occur largely as a result of purging and laxative abuse. We report here a study of the osmoregulation of plasma arginine vasopressin secretion and of vasopressin levels in the cerebrospinal fluid. This study was undertaken because vasopressin not only functions as the antidiuretic hormone, and thus as a principal modulator of fluid and electrolyte balance, but also because, in animals, centrally directed vasopressin delays the extinction of behaviors acquired during aversive conditioning. Thirteen normal-weight female patients with bulimia nervosa were studied after at least 1 month of nutritional stabilization and supervised abstinence from binge eating and purging. Plasma vasopressin, plasma sodium, and subjective thirst were measured serially before and during a 2-h infusion of 3% hypertonic saline (0.1 ml/kg min). In addition, cerebrospinal fluid was obtained by lumbar puncture upon admission and at 1 week before hypertonic saline infusion in 11 of these patients and in an additional 11 female patients who did not participate in the hypertonic infusion study. Fifteen healthy normal weight individuals (4 female, 11 male) served as controls for the hypertonic saline infusion and a separate group of 11 healthy normal weight female controls underwent puncture. Compared to controls, bulimic subjects showed a significant reduction in the plasma vasopressin response to hypertonic saline; in 12/13, plasma vasopressin correlated closely with plasma sodium, whereas in one patient vasopressin fluctuated erratically, with no relation to plasma sodium. Cerebrospinal fluid vasopressin levels were significantly higher in patients, and correlated positively with basal thirst level, which was enhanced in bulimics. Compared to controls, patients showed significant polyuria. We conclude that patients with bulimia nervosa have abnormal levels of vasopressin in their plasma and cerebrospinal fluid during abstinence from binge eating and purging. The disturbance in osmoregulation may aggravate the maintenance of adequate fluid volume in these patients, while the increase in centrally directed vasopressin may have relevance to their obsessional preoccupation with the aversive consequences of eating and weight gain.