Rats with hereditary hydronephrosis and littermates with normal kidneys were given 1% saline solution instead of drinking water, and renal excretory function, blood pressure, fluid volumes, and plasma renin activity were monitored. A marked increase in blood pressure and positive correlation between blood pressure and plasma volume were found in hydronephrotic rats after five weeks of salt loading, while animals with normal kidneys remained normotensive. No involvement of the renin-angiotensin system could be demonstrated in the observed phenomenon. It is suggested that the impairment of renal excretory function along with the deficiency of antihypertensive reno-medullary factors are responsible for the greater sensitivity of the hydronephrotic rats to the hypertensive stimulus of salt overload.