Background: Colonic cleansing with sodium phosphate causes intravascular volume contraction in some patients. This study tested the hypothesis that oral rehydration solution protects against intravascular volume contraction during colonic cleansing with orally administered NaP.Methods: Adult outpatients were randomized to ingest the regular clear fluid or an oral rehydration solution (ORS) during pre-colonoscopy purgation by ingestion of aqueous sodium phosphate. Clinical hemodynamic measurements and biochemical tests were obtained at baseline and after bowel preparation. Tolerability and colonoscopy visualization were assessed with questionnaires.Results: One hundred sixty patients completed the study. There were 80 patients receiving regular clear fluid (control group) and 80 patients receiving ORS (treatment group). By comparison with control, the treatment group resulted in significantly less intravascular volume contraction. The changes in orthostatic pulse (p<0.045), BUN/Cr ratio (p <0.002), urine specific gravity (p <0.000) were significantly greater in control than treatment group. The changes in biochemical parameters after purgation also suggested a greater degree of volume contraction in control than treatment group. Tolerability of the preparations was similar in both groups (p = 0.809).Conclusions: Oral rehydration solution protects against intravascular volume contraction during preparation for colonoscopy by ingestion of sodium phosphate. This approach is well tolerated by patients.