Kinetics of red cell washout, when isolated cat spleens are perfused with Ringer solution, show that the spleen corresponds to a three-compartment system. To determine whether or not there exist morphological counterparts to these compartments we examined microscopic sections from 16 spleens perfused by different volumes of Ringer solution. Red cells could be divided into three groups: (1) free cells in vascular channels and sinuses, (2) cells adhering to reticulum cells or sinus endothelium, and (3) cells in the cytoplasm of macrophages. When 50 ml were perfused no free cells were seen in vascular channels. After 600 ml few free cells remained in the sinuses. Thereafter the number of cells adhering to the sinus wall decreased gradually and cell counts agreed with predictions from the washout curve. We conclude that the compartments of our model (fast, intermediate, and slow) represent, respectively, free cells in vascular channels, free cells within sinuses, and cells adhering to sinus walls. The only cells trapped irreversibly are the very few found in the cytoplasm of macrophages. It is suggested that the slow compartment represents red cells in a pre-phagocytosed stage, e.g. aged cells, abnormal cells containing inclusion bodies, and possibly, reticulocytes in the process of maturation.