To determine value and limitations of mechanical cardiac stimulation by precordial thumps for termination of ventricular arrhythmias, we systematically treated 47 consecutive cases of ventricular tachycardias (resp. ventricular flutter or fibrillation) by this method. In 20 of 37 cases of ventricular tachycardias the arrhythmia was terminated by manual stimulation. The mean tachycardia rate amounted to 145/min (range from 102 to 222/min) in successfully treated patients. Bursts of rapid precordial thumps were more effective than single precordial thumps. In 17 of the 37 cases of ventricular tachycardia. The mean tachycardia rate was significantly higher (176/min, range from 120 to 250/min) than in successfully treated cases. Altogether, ventricular tachycardias with heart rate less than or equal to 160/min were terminated by mechanical stimulation in 17 of 22 cases, and ventricular tachycardias with heart rate greater than 160/min only in 3 of 15 cases. Ventricular fibrillation (n = 3) or ventricular flutter (n = 7) was not interrupted in any case by precordial thumps. In patients with ventricular tachycardia, mechanical stimulation extends the therapeutic possibilities. The rate of success is higher, the lower the tachycardia rate. The tachycardia rate is the only predictive parameter for therapeutic success.