The gastric tolerability of low dose conventional aspirin (acetylsalicylic acid) was compared with that of lysine acetylsalicylate in the dog. A total of 14 dogs were allocated to aspirin 250mg (n = 2) or 500mg (n = 4) tablets, lysine acetylsalicylate powder (LAS) corresponding to 250mg aspirin (n = 2) or to 500mg aspirin (n = 4), and to plain vehicle (n = 2). Animals were sacrificed after general anaesthesia. Macro- and microscopic examinations of gastric and duodenal mucosae were performed 3 or 5 hours after single dose oral administration. No significant duodenal lesions were found. On the gastric mucosa, vascular changes (oedema and microhaemorrhages) and foveolar atrophy were observed with a similar frequency and intensity in all groups receiving aspirin or LAS. Mucosal erosions with coagulative necrosis occurred only in the aspirin groups. Thus, the gastrotoxicity of aspirin at relatively low doses is apparent with a standard aspirin formulation, but seems substantially less when using a highly soluble acetylsalicylate salt.