1. The fall in blood pressure produced by a test dose of prazosin was greater in a group of patients with chronic renal failure than in a group with normal renal function.
2. This difference could not be attributed to increased reactivity, measured as the slope of the regression line relating mean blood pressure and plasma prazosin concentration, nor to retarded elimination of the drug.
3. The enhanced antihypertensive effect of prazosin in renal failure appears to reflect changes in the bioavailability or distribution of the drug, which result in higher drug concentrations for a given dose.