1. Po2, Pco2 and pH were examined in twenty-seven pulmonary ‘wedge’ samples of blood obtained from seventeen patients with various heart diseases.
2. High Po2 and pH, and low Pco2, values were found in comparison with those in simultaneous samples of peripheral arterial blood. In the pulmonary ‘wedge’ samples of blood, the following mean values were found: Po2, 17.65 kPa (132.2 mmHg); Pco2, 3.10 kPa (23.2 mmHg); pH 7.47. An explanation of these findings is considered.