BACKGROUND The Jackson Rees breathing system is commonly used for bag and mask ventilation in preschool children, although the lack of a pressure release valve can increase the risk of gastric insufflation. Therefore, we investigated the impact of bag and mask ventilation with a Jackson Rees system on functional residual capacity (FRC) and ventilation homogeneity and evaluated the effect of the level of training of the anesthesiologist in charge. METHODS Functional residual capacity and ventilation homogeneity were measured in 74 children (1-6 years) undergoing general surgery and the level of training of the anesthesiologist was recorded. FRC was measured (i) after intubation and (ii) after gastric emptying. Sixty-four children were ventilated using a Jackson Rees system, whereas 10 children were ventilated using a circle system to compare these two breathing systems in the second phase of the protocol. RESULTS Functional residual capacity and ventilation homogeneity increased in all patients following gastric emptying with the highest improvement (25%) being observed when nurse students were in charge of the ventilation with the Jackson Rees system. The lowest changes in FRC and ventilation homogeneity were observed when pediatric consultants were in charge, whereas ventilation by the pediatric nurse anesthetists led to significant gastric gas insufflation. However, the circle system was associated with significantly less gastric insufflation than the Jackson Rees system. CONCLUSIONS The efficacy of bag and mask ventilation was highly dependent on the training of the anesthesiologist with consultants demonstrating significantly better skills than any of the other groups. As the circle system is associated with a much steeper learning curve than the Jackson Rees system, its use in daily routine practice may prevent ventilatory impairment induced by gastric insufflation.