Blood pressure of indomethacin-treated and control rabbits was measured directly every hour in an automated device for 7-10 days. Indomethacin treatment was without effect, and the mean aortic pressure of all rabbits varied generally between 65 and 80 mm Hg. Since the indomethacin treatment was adequate to block the prostaglandin synthetase system, either circulating prostaglandin is not involved in blood pressure control or other regulatory mechanisms can readily compensate for absence of prostaglandin.