In the determination of an optimal rate for the delivery of stimulation pulses to a heart, the stroke volume (SV) of a heart ventricle is measured as a function of heart rate (HR), e.g. by determination of impedance in individual heart cycles whose duration is varied from the duration corresponding to the prevailing rate or the basic rate regarded as optimal for stimulation. The optimal rate should lie at the knee of this function. A corresponding knee is also found in the curve for cardiac output (CO). This rate is determined by calculation of an auxiliary function, e.g. HF=HR.times.CO, which displays a peak at the heart rate at which the knee is located, and the heart rate is determined which yields this maximum. The heart rate determined in this manner is subsequently employed as a basic rate.