An automated technique was developed for the measurement of capillary hematocrit in vivo. The method is based on the photometric detection of the passage of the red blood cells (RBC) through video photometric windows. Red blood cell flux is derived from the primary measurement of opacity and velocity, as opposed to frequency of cell passage and cell transit time. Consequently the technique is less prone to errors caused by the detection of joined cells. Tests in muscle capillaries, compared with hand count frame-by-frame analysis shows that the difference in measured flux is of the order of 16%. Fluctuations due to vasomotion are easily recorded by this approach.