Summary Groups of Mongolian gerbils were fed diets supplemented with 5 ppm sodium selenite, 12·8 ppm aflatoxin B1 or both. All groups receiving supplemented diets showed increased duodenal serotonin levels relative to the controls, indicating interference with serotonin metabolism by these agents both individually and combined. Aflatoxin B1 alone had the greatest effect and sodium selenite alone, the least; the mixture had an intermediate effect. The possible mechanisms of this interference are discussed.