The effects of dextran on the haemostatic system lead to prolongation of the bleeding time. To observe if the coating of endothelial cells by dextran influences the release of prostacyclin from the vessel walls in response to ex vivo perfusion, dextran 70 (1 g/kg b.w.) was given i.v. 15 min or 4 hours before excision of caval vein and aorta in rabbits, or was added to the perfusate. Vascular segments were perfused for 5 x 15 min, with the perfusate changed after each period and with arachidonic acid added on the last occasion. Prostacyclin was measured as its stable degradation product 6-keto-PGF1 alpha by radioimmunoassay. Arteries released more prostacyclin than did veins (p less than 0.01). Dextran given in vivo or ex vivo had no influence on release of prostacyclin from the walls of perfused arteries or veins. The anti-haemostatic properties of dextran 70, 1 g/kg b.w., thus do not seem to be mediated by prostacyclin release from vessel walls.