The hypothesis was tested that in young children when admitted to the hospital for acute illness, vasoactive hormone levels are raised. The plasma concentration of arginine vasopressin (AVP) and plasma renin activity (PRA) were measured in 103 acutely ill infants and children admitted to the hospital. Compared to 31 control children with elective surgery, plasma AVP and PRA levels were significantly elevated and plasma osmolality reduced in acute illness, indicating non-osmotic, cardiovascular AVP release. AVP and PRA elevations were found to be independent of the underlying diseases (e.g. respiratory infections, gastroenteritis, bacterial infections and the viral syndrome). Since cardiovascular AVP activation bears the risk of hyponatraemia in the case of hypotonic fluid therapy, initial fluid management should be performed with solutions containing half-normal or normal saline in acutely ill children.