To understand the secretion and synthesis of atrial natriuretic peptide we measured immunoreactive atrial natriuretic peptide from plasma, heart tissues and brain areas, and ANP mRNA was determined from heart auricles and ventricles of vasopressin-deficient Brattleboro rats (DI) and from desmopressin treated Brattleboro rats (DI+DDAVP). Long-Evans rats (LE) served as controls. DI+DDAVP rats were given for 3 days sc. injections of 0.5 micrograms 1-desamino-8-D-arginine vasopressin in 1 mL saline twice a day. The rats were housed in single metabolic cages and urinary output and water intake were measured daily. All the body and organ weight parameters were similar in the three groups when the rats were killed. No change was seen in the plasma ANP level between the groups. The right ventricle of DI+DDAVP rats had significantly (P