In 26 patients a direct comparison was made of systolic time intervals derived from the right and left external carotid pulse tracing and from the central aortic pressure, simultaneously recorded by means of a catheter-tip micromanometer. In addition, the delay time (required for pulse wave transmission to the neck) of the upstroke (DUEC) and incisura (DIEC) of the external carotid pulse tracing was studied. The influence of changes in heart rate on these intervals was evaluated by right atrial pacing. At resting frequencies DUEC exceeded DIEC by 4 to 4.5 msec (P less than 0.005). Increasing heart rate did not influence DUEC but prolonged DIEC. Differences between DUEC and DIEC were associated with a decrease and reversal with increasing heart rate (tau = 0.52, on the right carotid pulse; tau = 0.65, on the left pulse). Internally and externally measured left ventricular ejection time and pre-jection period were strongly correleated. Significant differences in absolute values were also observed. Whether these differences were positive or negative was dependent on heart rate. At rest external left ventricular ejection time was 4 to 4.5 msec (P less than 0.005) shorter, external pre-ejection period 5.5 to 6 msec (P less than 0.005) longer than corresponding intervals measured internally. At heart rates above plus or minus 100 beats/min the relationships were reversed. No significant differences could be demonstrated between these intervals measured on the right and left external carotid pulse.