Cardiopulmonary function was measured in 6 conscious dogs with progressive degrees of induced pneumothorax. Minute volume, respiratory rate, central venous pressure, systemic arterial pressure, pulmonary arterial pressure, pulmonary arterial occlusion pressure, heart rate, cardiac output, and arterial and mixed venous blood gases were determined before pneumothorax and at progressive volumes of pneumothorax equivalent to 50, 100, and 150% of the calculated lung volume. Tidal volume, pulmonary vascular resistance, alveolar to arterial O2 tension difference, physiologic dead space fraction, and pulmonary venous admixture also were calculated. Linear increases in respiratory rate, central venous pressure, alveolar to arterial O2 tension difference, and pulmonary venous admixture differed significantly (P less than 0.05). Linear decreases in tidal volume, pHv, pHa, PvO2, and PaO2 were also significantly different. Quadratic increases were significantly different for pulmonary arterial pressure and pulmonary vascular resistance. Trends were not significantly different for other values.