A flock of broiler chickens was monitored intensively for overt signs indicative of sudden death. Electrocardiographic tracing of terminal cardiac activity was obtained from six broilers succumbing to sudden death syndrome and from six control chickens which were killed by cervical dislocation. Normal sinus rhythm, heart rate and electrocardiogram (ECG) pattern with P, QRS, and T deflections were observed for two to three minutes in the control chickens. The ECG tracing from birds dying of sudden death syndrome showed no identifiable P, QRS, or T waveforms. Initial more regular oscillatory waves resembling ventricular flutter, changed to irregular, undulating waves characteristic of ventricular fibrillation. A straight line, indicating cessation of heart activity, appeared within four to six minutes of the onset of clinical signs. It was concluded that the sudden death syndrome is associated with an acute cardiovascular failure caused by a lethal cardiac dysrhythmia, ventricular fibrillation.