1. P o 2 , P co 2 and pH were examined in twenty-seven pulmonary ‘wedge’ samples of blood obtained from seventeen patients with various heart diseases. 2. High P o 2 and pH, and low P co 2 , 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: P o 2 , 17.65 kPa (132.2 mmHg); P co 2 , 3.10 kPa (23.2 mmHg); pH 7.47. An explanation of these findings is considered.