The effects of leukocyte-endothelium adhesion on microhemodynamics were studied in cat mesentery under control conditions and following tissue suffusion with the chemotactic agent N-formyl-methionyl-leucyl-phenylalanine (FMLP). The results indicate that under normophysiological conditions there is little or no leukocyte-endothelium adhesion in arterioles and venules. Tissue suffusion with FMLP significantly increases the number of adhering leukocytes in venules, but not in arterioles. Analysis of the number of adhering leukocytes (in venules) as a function of wall shear rate suggested that increased adhesion of leukocytes was primarily due to elevated adhesive forces and not the result of decreases in dispersal forces, i.e., wall shear stress. From measurements of upstream to downstream pressure drop, red cell velocity, and microvessel hematocrit in 16 unbranched venules, no significant changes in diameter (mean of 39.9 +/- 7.8 (SD) micron), intravascular pressure gradient (0.59 +/- 0.40 X 10(-2) cm H2O/micron), nor microvessel hematocrit (31.0 +/- 9.8%) occurred in response to FMLP. There were significant decreases in upstream pressure (8%) and estimated bulk flow (28%) as well as significant increases in the number of adhering leukocytes, from 1.5 +/- 2.8 to 11.4 +/- 8.3 cells/100 micron, and vessel resistance (81%). Changes in hemodynamics were found to be more pronounced in venules with small diameters. The observed response to FMLP suggests that changes in hemodynamics during leukocyte-endothelium adhesion can be accounted for by a decrease in the effective diameter due to obstruction of the lumen by WBCs, and that adhesive interactions between WBCs and endothelium are a major determinant of blood flow resistance in the microcirculation.