In 38 cardiac patients a direct comparison was made of systolic time intervals derived from the external carotid pulse tracing and from the central aortic pressure curve, simultaneously recorded by means of a catheter-tip micromanometer. In addition the transmission time of the upstroke and the incisura of the central pulse wave to the carotid artery was studied. In spite of close linear correlations, significant differences in absolute values were observed between internally and externally measured systolic time intervals. The transmission times of upstroke and incisura of the pulse wave also differed significantly. Both findings were directly related and dependent upon the heart rate.