Aspirin inhibits platelet function in many species. This study examined the effects of aspirin on platelet function in the pig, an economically important food animal and an animal model of human disease;Platelet function in man is partially age dependent with neonatal platelets routinely having a decreased platelet aggregation response. This does not appear to be true for the pig. Twenty neonatal piglets and 20 weanling piglets had the same or increased in vitro platelet response to platelet agonists as did 20 adult gilts and sows. An unusual finding was an occasional spontaneously reversible neonatal porcine platelet response to high-dose arachidonic acid;Commercially available oral aspirin, at various dosages, was administered to 20 weanling piglets. Collagen-induced platelet aggregation was maximally inhibited at an aspirin dose of 20 mg/kg. Aspirin inhibition could be overcome, however, by increasing the concentration of the collagen aggregating agent;Oral aspirin also decreased the cardiovascular response to intravascular infusions of collagen in 4 anesthetized piglets but did not completely prevent the increase in either pulmonary vascular resistance or in mean pulmonary arterial pressure. Oral aspirin pretreatment did, however, prevent the neutropenia that was observed in the 4 untreated control piglets which had received identical collagen infusions;It is concluded that the newborn pig has relatively mature platelet function and that aspirin can inhibit porcine platelet aggregation and platelet interactions with other mediators of inflammation. Aspirin therapy, therefore, may be clinically useful in some platelet-mediated diseases in the pig.