Aims Cardiac resynchronization therapy (CRT) by means of biventricular pacing (BiVP) is well established as a treatment for patients with heart failure (HF). Post-extrasystolic potentiation, (PESP) which involves a transient increase in myocardial contractility following a ventricular extrasystole, can be achieved using extrasystolic stimulation (ESS). On this basis, ESS has been proposed as a therapeutic. We assessed acute haemodynamic effects of ESS in the context of BiVP. Methods and results Patients (n= 15, left ventricular ejection fraction <40%, QRS ≥125 ms) with HF, received BiVP in combination with right ventricular (RV) ESS (single stimulus or pulse train). Left ventricular (LV) and peripheral arterial pressures were recorded and dP/dt was monitored. Addition of RV ESS to BiVP pacing led to a 21% increase in maximum (max) dP/dt (P< 0.001) and an 8.5 mm Hg increase in a systolic arterial pressure (P< 0.001). The modest fall in end-diastolic pressure (3.3 mmHg, P< 0.001) observed during ESS and BiVP was prevented by maintaining baseline sinus rate. Varying ESS modes or pacing outputs was not associated with differences in haemodynamic parameters. Conclusions Biventricular pacing in combination with ESS, with maintenance of sinus rate, improves myocardial contractility in patients undergoing CRT.