We have attempted to determine both experimentally and theoretically whether a significant quantity of oxygen enters the pulmonary blood before it reaches the alveolar wall capillaries. We built a microspectrophotometer that allowed us to record oxyhemoglobin saturation values with light reflected from transected frozen pulmonary arteries as small as 100 microns in diameter. We prepared anesthetized cats to provide optimal conditions for precapillary oxygenation, quickly froze their lungs with chilled liquid propane, and removed these lungs. We examined lung cross sections with the microspectrophotometer and recorded data indicating the presence of precapillary oxygenation. These data did not tell us how much this process normally contributes to total lung oxygenation because propane did not freeze the lungs fast enough to capture conditions exactly as they are in life. We therefore made several calculations to estimate the extent of precapillary oxygenation using available data on pulmonary arterial geometry and diffusing capacity. We concluded that pulmonary arterial blood may be as much as 15% oxygenated by this process at rest and as much as 100% during oxygen breathing.