A one compartment, mechanical model of the human lung-thorax system is presented and mathematically analyzed. The equation relating the thoracic muscular stress to the expired air volume is developed and investigated. Assuming that the pressure drop along the airways is a linear function of air flow rate and that the effective lung-thorax compliance is constant, a form for the muscular stress as a function of time is developed. This is used to predict volume-time and flow-volume curves, which are compared to those measured on a normal individual. It appears that these theoretical results have the essential characteristics of the experimental curves. These results, coupled with the one-to-one correspondence between the parameters of the model and those of the prototype, suggest that this model should have great utility in the study of ventilatory mechanics.