Background: Low volume oral colonoscopy preparations are easier to tolerate and are gaining popularity over large volume oral lavage solutions. Purpose: To compare the efficacy and tolerance of two low volume oral colonoscopy bowel preparations in a prospective randomized single-blinded controlled trial: NaP versus Mg. Methods: Ambulatory patients were randomized to receive one of two bowel preparations the day prior to colonoscopy. Group 1 received NaP 45 ml in a large glass of fluid at 10 AM and 7 PM. Group 2 received three 10-oz. bottles of Mg at 10 AM, 2 PM and 6 PM. Both groups were on clear liquids the day prior to colonoscopy. Patients responded to a questionnaire. The endoscopist blinded to the bowel preparation rated bowel cleansing as excellent, good or poor for the right and left colon.Volume of fluid suctioned was recorded. Results: Three hundred patients were randomized to NaP (n=160) or Mg (n=140). Both were well tolerated. See table below. Bowel cleansing was better overall with Mg, p < 0.05. Mg achieved a 94% and 97% excellent or good cleansing in right and left colon resp., compared to 73% and 89% for NaP. The volume of fluid suctioned was greater for Mg, 61cc (range 0-250) than NaP, 23 cc (range 0-150). Adverse effects were equivalent in both groups. Of patients who had both preparations, 62% favored NaP, 29% were indifferent, and 9% favored Mg. Of patients who had previously received large volume oral lavage solutions, the majority favored Mg or NaP. Rectosigmoid aphthous ulcers occurred in 6% with NaP and 1% with Mg. Conclusion: Mg and NaP were well tolerated and favored over large volume oral lavage solutions. Mg achieved more effective bowel cleansing, and less rectosigmoid aphthous ulcers than NaP, at the expense of a greater volume of suctioned fluid. NaP was an easier prep. Mg is an excellent alternative to NaP for low volume oral colonoscopy bowel preparation.