Dynamic pressure-volume methods with constant-pressure inflation are frequently used when assessing drug activity in the lungs of small animals where it is difficult to measure airway resistance and dynamic lung compliance. They all employ a ventilator which provides constant volume inflation and a breathing circuit designed to limit the airway pressure: the supply line from the ventilator to the animal is connected through a T-piece to a tube (immersion tube) immersed in a liquid-filled container. The immersion depth of the tube (backpressure) limits the peak inspiratory pressure (PIP). This approach should avoid the PIP within the circuit exceeding the backpressure. However, we have observed that a marked overpressure was generated during histamine-induced bronchoconstriction if a small diameter immersion tube (< 0.4 cm) was used when setting up such a circuit. In vivo as well as in vitro experiments were carried out with glass immersion tubes of different cross-sectional areas (0.3, 0.42 and 0.62 cm). These settings caused a significant difference in the duration of airway resistance changes among groups without affecting the peak overflow. Overpressure was dependent upon several factors: diameter of the immersion tube, tidal volume and ventilatory rate. Theoretical considerations, as well as in vitro and in vivo experiments, indicated that an immersion tube with inner diameter (ID) of 0.6 cm prevented the overpressure in the majority of experimental settings.