Intracardiac impedance measurements in conjunction with microprocessor controlled signal processing and lead technology provide a great variety of practical applications, particularly for physiologic control of the pacing rate of implantable pulse generators. As a prerequisite to obtain reliable systolic time intervals the impedance signal needs digital processing. Safety aspects and improvements in the signal to noise ratio were solved by an appropriate frequency band and synchronous demodulation. It is the aim of this paper to disclose a reliable method to determine the modified PEP, defined as the time interval between the onset of R-wave or the electrical exitation signal in pacing respectively and the maximum of the first derivative of the intracardiac right ventricular impedance curve. Details of the signal filtering as well as data processing are described including the use of a microcomputer circuit operating in a real time interrupt activated mode. Clinical data have proven the reliability of the unipolar intracardiac impedance plethysmographic methode. Measurements of left ventricular volume using an eight-pole intracardiac conductance catheter was first described by BAAN et al. [l]. Followed by the description of a four-pole electrode configuration [2] in the right ventricle (Fig. 1) allowing the online registration of SV, EDV, ESV and Systolic Intervals. To understand the factors that determine the relationship between intraventricular conductance and blood