The aim of this study was to investigate cell kinetic and morphological parameters in the intestinal tract of young (35 days old) and old (184 916 days old) germ-free (GF) and conventional (Conv) NMRI-KI mice of both sexes. Specimens from eight parts of the intestine were investigated microscopically. In the small intestine, the parameters determined included the percentage of cells in mitosis (i.e. mitotic index), the number of cells in the crypts and villi, the depth of the crypts, the height of the villi and the crypt-to-villus ratio (C:V). In the large intestine, the mitotic index (MI), the number of cells and the depth of the crypts were determined.RESULTS: Gender had a minor effect on the results. Cell kinetics: no effects were observed. Morphology : 12 out of 132 possible signi cant differences were found, half of which were located to the duodenum and the distal part of the colon (three each). No speci c tendency was found. Age had a stronger effect on the results than gender did. Cell kinetics: there were 4 out of 32 possible differences, all expressing higher MI in the small intestine of the young animals. Morphology : 20 signi cant differences were found, mostly related to there being more cells in the crypts in the young animals. Microbial status had the most marked effect on the results. Cell kinetics: 8 out of 10 signi cant differences were found in the large intestine, all expressing higher MI in the Conv animals. Morphology: there were 30 differences in the small and 25 differences in the large intestine. Most of the differences were related to the evidence of deeper crypts and more cells in the crypts, of the conventional mice compared to the GF animals, with the most pronounced results being in the large intestine.CONCLUSION: Gender, age and microbial status affected the intestinal cell kinetics and morphology in a compartmentalized manner, of which the microbial status of the animal showed the strongest effect.