A new instrument for the indirect noninvasive measurement of mean arterial pressure (m.a.p.) has been constructed and evaluated in man. The instrument does not require an external microphone or transducer and determines m.a.p. rather than systolic and diastolic pressure. Instead, the method employs the point of maximal oscillations as an indicator of m.a.p. The instrument automatically inflates a standard blood pressure cuff and determines the m.a.p. by measuring the cuff pressure oscillations as the cuff pressure is reduced by discrete increments. Cuff deflation in discrete increments, instead of continuously, allows the oscillation data obtained at each cuff pressure to be tested for artefacts and averaged, greatly enhancing artefact-rejection ability. The m.a.p. is selected as the lowest cuff pressure at which the oscillation amplitude is a maximum. The instrument was tested on the bicep and ankle in a series of 28 studies involving 17 human subjects with intra-arterial catheters. Averaging the mean errors from each of the 28 studies, there was an overall mean error of — 0·23 mmHg, with a standard deviation of 4·21 mmHg. The correlation coefficient was 0·98. The instrument was found to give good results in a wide variety of clinical subjects and physiologic states.