We subdivided regional erythrocyte (RBC) transit time into the fraction spent in the large (greater than 50 microns), small (20-50 microns), and capillary (less than 20 microns) vessels of canine lungs. Using a combination of physiological and morphological techniques, the number of RBC and granulocytes in each vessel size was obtained by dividing the aggregate volume of the cell by the calculated volume of a single cell. The data show that the average transit time was 0.60 s in large vessels, 0.10 s in small vessels, and 1.37 s in capillary vessels and was longer (P less than 0.05) in the upper lung regions for small vessels and capillaries. RBCs pass through approximately 60 capillary segments at an average velocity of approximately 550 micron/s. Pulmonary hematocrit averaged 0.95 of the systemic hematocrit and was lower in capillaries than small vessels. PMN's were concentrated approximately 10-fold in the small vessels and 100-fold in the capillaries compared with peripheral blood. The multisegmented nature of the capillary bed allows this concentration because RBCs find pathways to stream around the slower moving PMNs.