We studied to what extent dietary fiber may affect uptake, retention, and excretion of a food carcinogen (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, MeIQx) occurring in fried meat. Four diets--one fiber-free control and three containing either insoluble dietary fiber isolated from sorghum (100 g/kg) and wheat bran (100 g/kg) or the highly soluble pectin (50 g/kg)--were investigated. The fiber diets were given in amounts of 10 g/day to rats. Thus, each rat received 1 or 0.5 g fiber and 100 micrograms 2-14C-labeled MeIQx uniformly mixed in its daily diet. A 4-day adaptation period with unlabeled MeIQx was followed by a 5-day experimental period with 14C-labeled MeIQx, during which urine and feces were collected separately for analysis of radioactivity and mutagenicity. Furthermore the composition and the fermentability of the dietary fiber were determined. The present study shows that a diet containing fiber, especially fiber isolated from sorghum and wheat bran, affects the excretion pattern of the food carcinogen MeIQx in a manner suggesting a lower uptake and a decreased transit time through the gastrointestinal tract in a more diluted form than a nonfiber diet. Furthermore, less radioactivity was retained in the kidneys with sorghum and wheat bran than with the other two diets. On the other hand, none of these types of dietary fiber affected the retention of the hepatocarcinogen MeIQx in the liver 24 hours after the last oral intake. DNA adducts were formed to a higher extent in the kidney than in the liver. The highest levels were found in animals given the wheat bran diet.