The fastest circulation time, the mean circulation time, and the cortical mean passage time for cardio-green were measured in the dog kidney by a photoelectric dye dilution technique. At the same time measurements of the fastest circulation time and the mean circulation time for labelled red cells and plasma were performed by an external counting technique. The results obtained by the two methods showed good linear correlations. However, a significant difference between the fastest circulation times was found. The ratio between the two sets of values was 0.75, S. D. was 0.18, n=26. The difference was supposed to be due to the use of a too wide collimator opening. The ratio between the mean circulation times was 1.18, S. D. was 0.12, n=22. The mean circulation time measured by the photoelectric technique was the longest. Possible explanations for this discrepancy are discussed. The ratio between the mean passage time for dye passing through the cortex and the mean circulation time for red cells and plasma measured by the external counting technique was 0.54, S. D. was 0.10, n=19. This indicates that the graphical analysis and the mathematical model used for the external counting curve probably are correct, and that the mean circulation time calculated in this way is valid for blood passing the renal cortex.