These experiments were designed to characterize the nature and extent of diabetes insipidus present in a new model of genetic vasopressin (VP) deficiency, the Roman high avoidance rat homozygous for diabetes insipidus (RHA: di/di strain). The new strain was developed from an initial cross between Long-Evans derived Brattleboro (LE: di/di) rats and normal Roman high avoidance (RHA: +/+ ) rats, and has been bred to be congenic with the parent RHA: +/+ strain. RHA: di/di rats exhibited polydipsia, excreted dilute urine, and exhibited elevated plasma osmolality. RHA: di/di rats showed a similar urinary response to dehydration as LE: di/di rats. VP was undetectable by radioimmunoassay in the serum, brain, and neurohypophysis of RHA: di/di rats. VP-neurophysin containing cells were not observed in the brains of RHA: di/di rats upon immunocytochemical analysis. Thus, the new RHA: di/di strain exhibits essentially the same profile of diabetes insipidus as the LE: di/di rat. The congenic relationship between RHA: di/di and RHA: +/+ rats makes the RHA: di/di rat a useful model under circumstances where genetic variables unrelated to VP deficiency may confound the interpretation of data.