Although valproic acid absorption measurements have been reported for animal models, there have been no reports that correlate this to clinical experience with a patient. Reported here is the case of a patient who required twice the dose of valproic acid after colostomy for chronic constipation. The increased dosage required may have been due to rapid intestinal transit time or to bypass of the left colon. This case highlights the importance of gastrointestinal function in patients on valproic acid therapy.