Platelet aggregation in flowing venous and arterial blood during hypovolemic hypotension is followed by a previously described photoelectric method. It is shown that platelet aggregation can be inhibited by acetylsalicyclic acid (ASA) and by artificial ventilation. The inhibition of platelet aggregation by artificial ventilation is significantly more pronounced than that by ASA. That different times are needed for platelet aggregation in venous and arterial blood in dogs treated with ASA and with artificial ventilation is informative about the canine lung not only as a filter for aggregates but also as a source of aggregating substances.