The effects of acute and chronic water intoxication induced by the administration of oral water and arginine vasopressin (AVP) or 1-deamino-8-D-arginine-vasopressin (DDAVP) on blood acid-base equilibrium and aldosterone, corticosterone, and thyroxine secretion were studied in rats. Acute hyponatremia (3 hours) was associated with normal bicarbonate and blood acid-base equilibrium and a decrease in aldosterone and thyroxine concentrations, while corticosterone was increased. When similar levels of hyponatremia (serum sodium 110 mEq/L) were maintained for 24 or 72 hours, a normal serum bicarbonate concentration was observed, but blood acid-base equilibrium showed a mixed respiratory and metabolic alkalosis. Blood pH was negatively correlated with serum sodium concentration (R = -0.65; p < 0.001), as was the metabolic alkalosis (base excess; R = -0.64; p < 0.001) and the aldosterone concentration (R = -0.52; p < 0.01), while the PCO2 was positively correlated (R = +0.49; p < 0.01). Hyperaldosteronism was similar whether hyponatremia was induced with AVP or DDAVP and was observed even for mild hyponatremia. When hyponatremia was induced by a high water and salt intake (2.5% D-glucose, 0.45% NaCl; 15% body weight), aldosterone concentration was as high (about three times control values) as in rats with similar levels of hyponatremia but with a salt-free diet. The high salt intake was associated with a more severe metabolic alkalosis (base excess +5,5 mEq/L). In chronic hyponatremia, corticosterone and thyroxine values were normal. In hyponatremia related to syndrome of inappropriate secretion of antidiuretic hormone, the normal serum bicarbonate level is an expected observation; as in acute water intoxication, it stays normal.(ABSTRACT TRUNCATED AT 250 WORDS)