We developed a new indirect method for the measurement of sinoatrial conduction time (SACT) and the sinus node return cycle (SRC) with a transvenous catheter technique. Two early premature stimuli, at intervals 50 msec longer than the effective refractory period (ERP), were given to the right atrium. These early stimuli were followed by eight constant stimuli. The interval of the constant stimuli was a little shorter than the basic cycle length (BCL). The return cycle A1Ar was measured and plotted on the abscissa; the next interval ArA3, was measured and plotted on the ordinate. This was called the "base point". A new stimulus, A2, was then added to the train of stimulations, first at a point simultaneous with Ar. It was then shifted toward the last constant stimulus at 10-20 msec intervals until A2 met the ERP. The relationship between A1A2 and A2A3 was obtained by the repetition of the procedures with various A1A2 intervals. It had two zones, compensatory and non-compensatory. We postulate that the atriosinus conduction time of the last of the eight stimuli was equal to that of A2 when the stimulus A2 first captured and reset the sinus nodal pacemaker cells, as indicated by the transition point of the two zones. Based on this supposition, SACT and SRC could be measured as the intervals from the base point to the transition point and from the transition point to the eighth stimulus, respectively.