Colchicine and methotrexate are commonly used in the treatment of gout and rheumatoid arthritis, respectively; however the mechanism(s) of action of these drugs remain(s) unknown. The objective of this study was to determine whether colchicine and methotrexate can modify the adhesion and emigration of leukocytes in postcapillary venules that are exposed to inflammatory mediators such as platelet-activating factor (PAF) and leukotriene B4 (LTB4). The rat mesentery was prepared for in vivo microscopic observation. Venules with internal diameters ranging between 25 and 35 microns were selected for study. Erythrocyte velocity, vessel diameter, leukocyte rolling velocity, and the number of adherent (stationary for greater than or equal to 30 sec) and emigrated leukocytes were measured during superfusion of the mesentery with bicarbonate-buffered saline (BBS). Repeat measurements of adhesive and hemodynamic parameters were obtained between 50 and 60 min of superfusion with either 100 nM PAF or 20 nM LTB4 added to the superfusate. In some experiments, 1 microM of either colchicine or methotrexate was added to the superfusate containing either PAF or LTB4. Both PAF and LTB4 caused increases in leukocyte adherence and emigration and reductions in leukocyte rolling velocity and venular shear rate. Colchicine effectively prevented all of the adhesive and hemodynamic alterations induced by both inflammatory mediators, while methotrexate was largely effective in preventing the responses elicited by PAF, but not LTB4. These results indicate that the therapeutic actions of colchicine and methotrexate may result from the ability of these agents to interfere with the adhesion and emigration of leukocytes from postcapillary venules.