1. The influence of the intact kidney on blood pressure, extracellular fluid volume (ECF, ferrocyanide space) and ECF volume distribution was studied in rats 60 days after constriction of the contralateral renal artery. 2. Renal artery constriction increased both blood pressure and ECF levels. The rise of both variables was more pronounced in rats with the contralateral kidney removed (one-kidney rats) than with the contralateral kidney intact (two-kidney rats). 3. The interstitial fluid volume increased similarly in both experimental groups, the more pronounced ECF increase in the more severly hypertensive one-kidney rats being due to increased plasma volume. In the less hypertensive two-kidney group the plasma volume was not increased significantly but correlated positively with the blood pressure levels. 4. The plasma volume/interstitial fluid volume ratio was decreased in the two-kidney group but did not differ from control values in the one-kidney group thus resembling the ECF partition reported in the hypo-and hypervolaemic type of essential hypertension, respectively. 5. It is suggested that the antihypertensive influence of the intact kidney may be partly due to its ability to escape from the sodium-retaining mechanism activated by renal artery stenosis, which determines the degree of plasma volume expansion and, in connection with this, also the degree of blood pressure elevation.