The relation between dose, systemic availability, and response to oral debrisoquine was studied in 13 hypertensive patients receiving no other treatment. In 11 who received the same daily dose (40 mg) the fall in mean standing systolic blood pressure varied between 0-3 and 44-4 mm Hg. There was a ninefold difference in the daily urinary excretion and pre-dose plasma concentration of unchanged drug but an inverse correlation between daily urinary excretion of debrisoquine and its 4-hydroxy metabolite (r= -0-86), suggesting that a low recovery of debrisoquine occurs because of extensive metabolism. There was a significant correlation between the fall in standing systolic blood pressure and the mean daily urinary excretion (r= +0-82) and pre-dose plasma concentration (r= +0-82) of unchanged debrisoquine. In contrast, there was a significant inverse correlation between the urinary recovery of the metabolite and the fall in blood pressure (r= -0-82). The availability of debrisoquine is the major determinant of response to this drug. In the absence of side effects a poor response may be an indication to increase the daily dose rather than add another hypotensive agent.