A study was carried out to find out the relationship between arginine vasopressin (AVP) release and plasma osmolality in 15 young men (age: 21.4 yr). After an overnight fasting, wale. (20 ml/kg) was imbibed, and venous blood and urine samples were collected every 30 min for 90 min. then 5% saline was infused (0.06 ml/min/kg) for 120 min. AVP was extracted on Sep-Pak column and measured by radioimmunoassay. Under basal condition, plasma osmolality (pOsm), AVP (pAVP) and aldosterone (pAldo) levels were 286.5 mOsm/kg, 1.1 pg/ml, and 140 pg/ml, respectively. pAVP became undetectable during maximum water diuresis, and increased in response to hypertonic saline infusion. pAVP level began to increase when pOsm was above 280 mOsm/kg. Changes in urinary AVP excretion (uAVP) was parallel to pAVP levels. The fall in pAVP was followed by a decrease in uAVP, uOsm and an increase in free water clearance, while the later rise in pAVP was followed by an increase in uAVP, uOsm and a decrease in free water clearance. When pooling all data together, relationships between pAVP and pOsm, and uAVP and uOsm were best expressed by an exponential relationship (r=0.78, 0.86, respectively). pAldo level decreased to 71 pg/ml after water ingestion, and decreased further to 30 pg/ml 2 hr after 5% saline infusion. Even at the same pNa, pAldo level during dehydration state was significantly higher than during hydration state. Negative exponential relation (r=-0.59) was observed between pAldo and pNa. Response to change in body fluid volume was greater in aldosterone than in AVP release.