A study based on sodium depletion, sodium loading and indomethacin treatment was conducted on rats. After sodium depletion significant activation of renin-angiotensin-aldosterone system and insignificantly increased kallikrein excretion without changes in prostaglandin excretion and plasma vasopressin were found. After sodium loading significant decrease of PRA (-83%), prostaglandin excretion (-34%), insignificant decrease of aldosterone and kallikrein excretion and significant increase of vasopressin (134%) were established. Indomethacin (5 mg/kg b.w.) administered for 8 days showed an antidiuretic and an anti-natriuretic effect, inhibition of PG-synthesis by 57% and of PRA by 68%, a significant increase of plasma volume (21%), blood volume (24%) and mean arterial pressure (14%). The relationships between vasopressor and vasodepressor hormonal systems in the rat are discussed.