During gravidity lizards experience a striking decrease in lung volume as a result of lung compression by eggs growing within the body cavity. In order to understand the effect of this decrease in lung volume on the respiratory biology of gravid egg-laying lizards, we measured changes in total lung volume, resting and postexercise expired volume, minute volume, respiratory frequency, and carbon dioxide production rate during reproduction in the Collared Lizard, Crotaphytus collaris, and the Leopard Lizard, Gambelia wislizenii. We found that compression of the lungs by shelled eggs resulted in an average 48% (range: 26–70%) decrease in total lung volume compared to the same postlaying C. collaris females, and an average 38% (range: 29–46%) decrease in G. wislizenii. CO₂ production rates were altered significantly during reproduction in female C. collaris and were 58% higher in females carrying late-stage follicles, compared to after laying. Despite the remarkable reduction in lung volume in both of these species and the increase in CO₂ production rates in C. collaris, no ventilation parameters changed over the course of reproduction. The highly distensible body cavities of C. collaris and G. wislizenii appear to be able to accommodate both growing eggs and adequate lung volumes for normal respiratory function during gravidity.