The influence of the phase of ventilation on the transthoracic impedance and defibrillation success was studied in 6 mongrel dogs. Ventricular fibrillation was induced by a transvenous bipolar catheter electrode. Defibrillation was attempted after 1 min of ventricular fibrillation. The initial stored energy levels were 20 watt-sec for the 1st two shocks, one delivered in inspiration and one in expiration. If the shock at the initial energy level was not successful, the energy level was increased by 10 watt-sec before the next energy level shock. Initial discharge was given at inspiration in half of the animals and at expiration in the other half. The transthoracic impedance to defibrillator discharge was measured with each shock.The study revealed a significantly higher transthoracic impedance with inspiration (76.3 ± 13 ohms versus 68.4 ± 12 ohms expiration, p < 0.01), and a significant decrease in defibrillation success rate when shocks were delivered in inspiration (10%) compared to expiration (50%). The phase of ventilation is an important determinant of transthoracic impedance to defibrillator discharge and has a significant influence on defibrillation effectiveness.