The physiological basis for the polyuria and polydipsia occurring in some manic-depressive patients treated with lithium salts was studied in vivo and in vitro. Three lithium-treated polyuric patients, in whom other causes of a concentrating defect were excluded, had abnormal urinary concentrating abilities after a standard water depreviation test. Two of these patients failed to respond to exogenous vasopressin (ADH) and one had a subnormal response. The abilities of these patients to excrete solute-free water (CH2O) was comparable to normal subjects during steady-state water diuresis, suggesting no gross abnormalities in sodium transport. However, each of these patients demonstrated abnormally low capacities to reabsorb solute-free water (TCH2O) under hydropenic conditions after administration of hypertonic saline and vasopressin. These in vivo findings demonstrate at least a nephrogenic basis for the diabetes insipidus syndrome manifested by these three patients.