This study tested the hypothesis that administration of immunosuppressive doses of prednisone in conjunction with ultralow-dose aspirin (0.5 mg/kg/day) would result in gastroduodenal lesion scores similar to those found in dogs administered only immunosuppressive doses of prednisone, but that the gastroduodenal scores from both of these treatment groups would be significantly higher than placebo when administered to healthy dogs for 27 days. Eighteen healthy adult purpose-bred dogs were divided randomly into three groups. Group I received placebo capsules and placebo suspension, Group II received prednisone capsules (mean 2.3 mg/kg, range 2.0-2.4) and placebo suspension, and Group III received prednisone capsules (mean 2.3 mg/kg, range 2.3-2.5) and aspirin suspension (0.5 mg/kg) by mouth once daily for 27 days. Gastroduodenoscopy was performed on days -7 (baseline), 5, 14, and 27 of treatment. Four regions of the stomach (angularis incisura, body, pylorus, and cardia) and the proximal descending duodenum were systematically scored on a scale of 1 (normal) to 11 (perforating ulcer) by an experienced observer who was blinded to the treatment groups and clinical signs of each subject. Dogs were observed every 8 hours for vomiting, diarrhea, and inappetence. Feces were scored on a scale of 1-5 with diarrhea defined as a fecal score <4. Lesion scores for each group, at each location, and total scores, at each time period were evaluated for the effects of time and treatment using a Kruskal-Wallis test. Total dog days of vomiting and dog days of diarrhea in each group were compared using a Wilcoxon rank sums test. Significance was determined at p<0.05. There were no significant differences in median total gastric lesion scores between any of the groups at any time during the study. There was no location effect on regional gastroduodenal lesion scores and there was no significant change in gastroduodenal lesion scores over time in any of the groups during treatment. Significantly more dog-days of diarrhea occurred within the prednisone and aspirin group during the experimental period (Period 2) in comparison to Period 1. However, no significant differences were found between any of the groups for dog-days of vomiting, diarrhea or inappetence at any time in the study. / Master of Science