Assessment of the gas volume that actually reaches the airways during mechanical ventilation appears to be a difficult task because of the presence of the breathing circuit. Most ventilators measure tidal volume at ventilator level making the determination of circuit compliance a critical factor in estimating the actual tidal volume. Tubing compliance can be measured in several ways and its value, being strongly dependent on the compressibility of the gas, may significantly differ depending on the measurement procedure. This paper addresses the dependence of the circuit compliance on the flow rate, and a theoretical hypothesis on the pneumatic behaviour of the breathing circuit is formulated and experimentally validated, with several tests conducted in vitro on an infant analogue. The dependence of the compliance on the inspiratory flow is experimentally assessed, and differences of about 20% on the measured value in the common flow range utilized in infant ventilation have been found, with consequent estimation errors of the volume delivered. Experimental tests show that the correct value of the tidal volume actually delivered to the patient can be reliably estimated from measurements performed at the ventilator level if the circuit compliance is determined with the same flow rates that will be utilized in mechanical ventilation.