In an attempt to evaluate the possible role of mutations in the age-dependent increase of tumor incidence, we studied the mutational burden that accumulates in the aging process in different parts of the digestive tract in mice. The mutations were monitored in lacZ genes integrated in the mouse genome. The digestive tract was divided into the esophagus, stomach, proximal, medial, and distal part of the small intestine, and the colon. Epithelial tissues were separated from these tissues with the exception of the esophagus, in which case the whole tissue was examined. At a young age, the mutant frequencies as well as the molecular nature of the mutations were similar among the tissues examined. In old age, on the other hand, mutant frequencies were elevated to different degrees among the tissues; they were high in the small intestine and colon, intermediate in the stomach, and low in the esophagus. The molecular characteristics of the mutations also revealed distinct tissue-specificity; there were elevated rates of a small deletion mutation in the esophagus, G:C to T:A transversion in the proximal small intestine, and multiple mutations in the distal small intestine and colon. The results indicate that different parts of the digestive tract suffer from different kinds of mutational stress in the aging process. The nature of the multiple mutations suggests the presence of a mutator phenotype based on an imbalance in deoxyribonucleotide pools.