During a 48-day period, 12 male dogs received four buffered sodium acetylsalicylate formulations, quantitatively virtually identical (a homogenenous disintegrating swallow tablet, a swallow tablet in which the sodium acetylsalicylate was contained in a dissolving core, an encapsulated powder, and an aqueous suspension), at 650-mg aspirin equivalent doses twice daily during four 7-day treatment periods (each preceded by a 5-day period of no treatment) in a complete changeover fashion. Mean daily fecal blood losses of 0.75, 1.37, 1.43, and 2.89 ml were observed in the 12 dogs during treatment with the aqueous suspension, the homogeneous tablet, the encapsulated powder, and the core tablet, respectively. These findings indicate that the physical form of buffered acetylsalicylate formulations is a critical factor in the effect of such formulations on GI microbleeding.