We measured the resistance in nine complete ventilator circuits, partial circuits and 7, 8, and 9 mm ID endotracheal tubes at flow rates of 20 to 120 liters per minute. We found a statistically significant (p less than 0.01) increase in resistive pressure with increases in flow rate, as the diameter of the ETT decreased, and as each component of the ventilator circuit was added to the ETT. There was a curvilinear increase in resistive pressure to increase in flow rate. However, when resistances were computed, the Bennett cascade "circuit" created higher resistance at 20 lpm than at flow rates up to 120 lpm. The Bennett cascade humidifier added the greatest resistive pressure, 3.5 to 8.5 cm H2O, the Engstrom Edith, 0.5 to 6.5 cm H2O, and the Conchapak added the least, 0.0 to 2.5 cm H2O at flow rates of 20 to 120 lpm. After all the components of the ventilator circuit were attached to the ETTs, there was approximately a 97 to 450 percent increase in resistive pressure compared to the resistive pressure created by the ETTs alone.