Using a mathematical model, the authors have confirmed the finding of Salanitre and Raekow that the rate of rise of alveolar halothane invivois more rapid in children and infants than in adults. The reasons advanced by Salanitre and Rackow for the more rapid approach to equilibrium include greater perfusion and ventilation and diversion of a greater fraction of cardiac output to highly perfused tissues. These partially explain the more rapid increase in the young. Additional explanations relating to differences between the experiments in children and adults are necessary to reconcile findings invivowith simulation findings. Subanesthetic halothane concentrations were used in adults, while anesthetic levels were used in children. The depression of cardiac output and consequent reduction of uptake produced by anesthetic levels contributed to the more rapid increase in children. In addition, metabolism of halothane at subanesthetic levels probably caused a greater fraction of the inspired halothane to be degraded in the adults, thereby slowing the increase in alveolar concentration.