1. The relationship between various body fluid compartment volumes, plasma renin activity and mean arterial blood pressure was studied in twenty-six patients with chronic renal failure.
2. Mean arterial blood pressure was positively correlated with total exchangeable sodium, blood volume and plasma renin activity: there was no significant correlation with either total body water or extracellular fluid volume.
3. Multiple regression analysis revealed that plasma renin activity combined with total exchangeable sodium, blood volume, red cell mass or total body water provided a better means of predicting blood pressure than any of the variables taken alone.
4. In a second study performed after a period of regular dialysis treatment no correlation was found between mean arterial pressure and either body fluid compartment volumes or plasma renin activity.