Pretreatment of rat large vein endothelial cells (ECs)in vitrowith cytochalasin B (CB, 1×10−5M) significantly reduced the narrowing of these cells in response to subsequent exposure to platelet-activating factor (PAF, 2×10−6M) or the calcium ionophore A23187 (2×10−5M). PAF (5×10−6M) also caused leakage of colloidal carbon (CC) into the walls of microvessels of the rat small intestinal vasculature when included in a gelatin-containing artificial perfusatein vitro. Pretreatment of the vessels with CB (1×10−6M) or phalloidin (1×10−7M) reduced this CC labelling. The results obtained provide evidence for the involvement of actin filaments in the EC shape changes, or“contractions”, that are thought to occur in inflammatory conditions.