An index of peripheral vascular resistance, derived from directly measured arterial pressure, has been developed and tested in an animal model. Assuming a mono-exponential character for diastole, the time constant for direct arterial pressure decay in the central aorta was obtained with high-fidelity recording techniques from six anaesthetized dogs. Changes in peripheral resistance were induced by infusion of phenylephrine or nitroprusside. The time constant correlated well with resistance determined from 1-min averages of pressure divided by integrated electromagnetic flow (r = 0.845). A portable system used for direct arterial pressure recording in ambulant outpatients yielded a mean correlation of 0.822 with resistance. In a study of patients undergoing cardiac catheterization, a time constant obtained from the brachial artery (the site of pressure recording in the ambulant subjects) correlated well with the aortic time constant (n = 6, r = 0.964). The results of the present work suggest that the time constant derived from direct arterial pressure waveform may be a useful index in the evaluation of the peripheral vascular resistance. Furthermore, with the help of the portable system it might be used for the continuous monitoring of resistance changes in ambulatory subjects.