Spirometers are useful for enhancing students' understanding of normal lung volumes, capacities, and flow rates. Spirometers are also excellent for understanding how lung diseases alter ventilation volumes. However, spirometers are expensive, complex, and not appropriate for programs with limited space and budgets. Therefore, we developed a simple, inexpensive, small model of a spirometer. Activity-based models are more valuable for enhancing learning than many hours of passive instruction. The model spirometer enables students to measure ventilation volumes as well as simulate lung diseases and positive pressure (mechanical) ventilation. The spirometer consists of a glass 5-ml syringe connected to the "tracheal tube" of an existing model [(1), Fig. 1]. A glass syringe is used because the plunger slides with less resistance. The spirometer must be filled with air before it is attached to the tracheal tube. When the plunger of the lung apparatus ("diaphragm") is pulled down, the air contained in the spirometer flows into the balloon ("lung"). Conversely, when the diaphragm is pushed up, the air flows from the lung into the spirometer. This volume, "tidal volume," can be measured. In addition, the number of "breaths" per time, "respiratory rate," can be determined, and minute ventilation can be calculated by multiplying tidal volume by respiratory rate. By use of this approach, students are able to determine lung volumes, capacities, and flow rates. Furthermore, the effects of obstructive and restrictive lung diseases can be simulated. An obstructive lung disease can be simulated by placing a clamp on the tracheal tube. A restrictive lung disease can be simulated by limiting the range of the diaphragm movement. Finally, mechanical ventilation can be simulated by using the plunger of the spirometer, forcing air to move into and out of the lung. This simple addition to an existing model (1) enhances students? understanding of ventilation volumes.