Positive urine and blood assays of methylene and leukomethylene blue were obtained from 9 volunteers with normal colonic and renal function after a 100 ml, methylene blue enema (50 mg.). The study conclusively demonstrates that methylene blue is absorbed by rectal mucosa and excreted by the kidneys as a colored dye in the urine. Thus, intrarectal methylene blue cannot be used to confirm the diagnosis of a colovesical fistula.