&NA;The authors compared two groups of patients receiving oral 5% mannitol for bowel preparation to see whether the volume of rectal effluent and quantity of bowel preparation could be maintained by a smaller oral mannitol intake supplemented by an intravenous infusion. Nineteen patients drank 2‐3 1 5 % mannitol, supplemented by an intravenous infusion of N/saline (intravenous Group) and 19 patients drank 4‐5 1 5 % mannitol (oral group). The volume of rectal effluent and the quality of bowel preparation was the same in both groups. The loss of sodium in the oral group was corrected by the intravenous infusion, but the infusion resulted in greater water retention. There was no reduction in the incidence of vomiting between the two groups.