Fecal blood volume was determined daily in 11 dogs with single gastric ulcers. Beginning 11 days after production of the ulcers and dogs received, in crossover fashion, 2 placebo or ordinary 325-mg aspirin tablets orally twice daily during two 7-day treatment periods separated and followed by 5-day periods of no treatment. Mean daily fecal blood volumes of 0.52 and 3.25 ml were observed during periods of treatment with placebo and aspirin, respectively. However, in 7 previous studies in this laboratory a total of 24 normal dogs have received 7-day courses of treatment with 650 mg ordinary aspirin twice daily on 105 occasions; during these 105 treatment periods fecal blood volume averaged 2.90 ml/day. Thus, it is concluded that the effect of ordinary aspirin in dogs with gastric ulcers is essentially the same as the effect in normal dogs, and that there is no tendency for dogs with gastric ulcers to bleed massively in response to aspirin.