BACKGROUND: A clean bowel preparation prior to X-ray examination is essential to obtain an accurate diagnosis. OBJECTIVES: This study was performed to compare the effect of senna and polyethylene glycol on the bowel preparation of dogs before abdominal radiography. METHODS: Six adult healthy mongrel dogs from each sex ranging in age from 18 to 30 months received either 4 or 8 g/kg polyethylene glycol or 20 mg/kg senna through an orogastric tube 3 weeks apart. Food was withheld approximately 12 hours prior to drug administration. Radiography was performed 12 hours after bowel preparation. Only water was given ad libitum to animals during the day before radiography. Radiologist who was unaware of the method of bowel preparation, reviewed the standard radiographs and graded the feces and gas in the colon and rectum, fecal radio-opacity and quality of radiographs on a scale of 0 to 3 (0=inadequate to 3=excellent). Dogs were observed for any drugs side effects up to two weeks after administration of the laxatives. RESULTS: The cleanliness scores of colon for the 4 and 8 g/kg polyethylene glycol and senna groups were similar, 2.17±0.41. Also, the cleanliness scores of rectum for the 4 and 8 g/kg polyethylene glycol and senna groups were 2.50±0.55, 1.83±0.75 and 2.50±0.55, respectively. Both polyethylene glycol dosages and senna significantly improve bowel cleansing score in dogs (p<0.05). There were no significant differences between the groups with respect to colon cleansing scores, fecal radio-opacity and quality of radiographs (p≥0.05). 8 mg/kg polyethylene glycol significantly reduces feces impaction in rectum versus 4 mg/kg (p<0.05). There were no clinical differences between the two groups for any of the side effects. CONCLUSIONS: It is concluded that 8 mg/kg polyethylene glycol and senna are safe and effective methods of colon cleansing, well tolerated by dogs.