In three isolated organs, spleen, cardiac and skeletal muscles, kinetic studies of red cell washout were carried out by using perfusion of the cell-free, oxygenated Ringer's solution. It is found that in each organ there are slow components for red cells to be emptied out from the vascular lumens ranging 30 to 50 minutes as the desaturation half-time. The slowest decay constants (K) are -1.48×10^-3 for spleen, -2.33×10^-3 for gastrocnemius muscle, and -4.0×10^-3 for cardiac muscle.