Among seven schizophrenic patients subject to water intoxication (six men and one woman, mean age 39.1 +/- 6.9 years), we measured serum sodium, plasma arginine vasopressin, and urine osmolality at 7 a.m. and 4 p.m. on eight consecutive Thursdays. On the days of greatest diurnal change in serum sodium, the 7 a.m. serum sodium was 141.1 +/- 1.8 mEq/l and the 4 p.m. value was 129.9 +/- 3.2 mEq/l. Plasma vasopressin also tended to be lower at 4 p.m. but, in many cases, was inadequately suppressed for the level of hyponatremia. The urine was dilute at both 7 a.m. and 4 p.m. and mean urine osmolality did not differ at the two times. In three patients, urine osmolality was consistently subnormal relative to plasma vasopressin at both 7 a.m. and 4 p.m. This abnormality was consistent with nephrogenic diabetes insipidus secondary to lithium and, possibly, phenytoin which the patients received to protect them against hyponatremia. We conclude that the combination of polydipsia and abnormal osmoregulation of vasopressin secretion contributes importantly to the afternoon hyponatremia found in schizophrenic patients subject to water intoxication.