The Iuminal surface of canine jugular and femoral veins was exposed to Hypaque 50% or Conray 60 under three different conditions. In Group I the contrast media were administered as a slow flow through injection via a side branch. In Group II contrast media were injected via a side branch and venous stasis induced by percutaneous compression of the downstream end of the external jugular vein for 7 minutes prior to local dissection for exposing the vein. In Group III the local dissection for exposing the vein was performed before injection of contrast media and induction of stasis. Controls for each group consisted of similar procedures in the absence of contrast media. In Group I, contrast media caused little or moderate adhesion of leukocytes in 5 of 6 cases, but heavy leukocyte invasion in one vein. In Group II, contrast media caused an increase in the extent of leukocyte adhesion and invasion in 2 of 5 veins of paired sets. In Group III, contrast media increased the extent of leukocyte adhesion and invasion in 6 of 11 veins of paired sets. In the other 5 sets the control and contrast-exposed veins were similar. These findings suggest that contrast media may exaggerate the adhesion to and invasion of the venous wall by leukocytes when surrounding tissues are damaged and stasis of several minutes' duration is induced.