Colonic levels of nitrite and circulating levels of nitrate are elevated in subjects with chronic diarrhea. The role of colonic epithelial cells in oxidation-reduction of nitrite and consequent control of nitrite and nitrate levels is unknown. Isolated rat colonocytes and isolated loops of colon were used to study oxidation and reduction of nitrate and nitrite. Colonocytes oxidized nitrite to nitrate at a rate of 162 +/- 33 nmol/min.g (dry wt) (n = 6) over 0-20 min, a value increased by addition of 5 mM glucose and 1 mM nicotinamide adenine dinucleotide (p = less than 0.01), but not altered by antibiotics. Nitrite oxidation correlated linearly with the dry weight of isolated colonocytes (r = 0.92), indicating a cellular reaction. Nitrite was absorbed at a rate of 0.28 nmol/min.cm2 (n = 9) from the colonic lumen, which reflected 74% absorption of the available nitrite. Of the absorbed nitrite, 14.3% reappeared in the colonic lumen as nitrate. Colonocytes may be important in oxidizing nitrite to nitrate, thus preventing excess nitrite from entering into the circulation and returning some nitrate to the colonic lumen for respiration by anaerobic bacteria. The interaction of nitrites and colonocytes deserves further study with regard to the oxidation-reduction of nitrite and to the development of colonic neoplasia in chronic diarrheal conditions.