Intracardiac blood oxygen saturation was determined in 31 patients, at cardiac catheterization, by means of a fiberoptic in vivo hemoreflection system, giving an immediate report. When compared to in vitro spectrophotometric analysis of samples obtained through a second catheter, a standard error of estimate of only 1.99-per cent saturation (r = 0.966) was found for the higher speed instrument (response time 0.07 second) and 1.13-per cent saturation (r = 0.992) for the slower instrument (response time 1.5 seconds).The average of the standard deviations of saturation values in cardiac chambers in which no primary mixing occurred, was only 0.88-per cent saturation. The continuous recording of oxygen saturation while moving the catheter, permits measurements at multiple sites within a brief period of time (14 to 26 seconds through the right side of the heart). Oxygen saturation changes can be continuously monitored under changing conditions, such as exercise. Finally, the higher speed instrument permits investigation of changes in oxygen saturation in patients with congenital heart disease within portions of the cardiac cycle.