Transient generalized oedema was observed in a patient upon exposure to a hot climate. Oedema disappeared in 5 days. Clinical study during oedema revealed decreased urine volume, high urine osmolality, high urine sodium concentration, increased blood volume, hyponatraemia and hypo-osmolality. The total solute excretion was unchanged. The endogenous creatinine and para-aminohippurate clearances were normal. The findings were suggestive of increased antidiuretic hormone activity, and heat might be responsible.