A simple and novel technique that utilizes the zero-crossing points of the first time derivative of intra-ventricular pressure (dP/dt) to mark systole, is proposed. Discrete differentiation of the sampled pressure waveform is calculated using a difference equation. Filtration of high-frequency noise in dP/dt is achieved using a low-pass Butterworth filter of order 4 and a cutoff frequency of 10 Hz. The filter is realized digitally using infinite impulse response filter stages. Double filtering of discrete dP/dt is used to eliminate time shifts. The methods are evaluated on data obtained from six large, white, anaesthetised and open chest pigs, instrumented with a conductance catheter. The zero-crossing points of the filtered dP/dt compare very well with the R-waves of the electrocardiogram (ECG) as markers of systole. The mean error is 1.3% of the duration of the heartcycle. Significantly, our results provide a solution to a problem often encountered with multiuse pressure-volume catheters when an ECG signal cannot be obtained. In this situation, the zero-crossing points of dP/dt, rather than the R-waves of the ECG, can be used as a marker of systole, thus enabling the construction of end-systolic pressure-volume relations to assess cardiac contractility.