The aim of the study was to characterise caecal indices in rats fed diets supplemented with high doses (0.4, 0.8 and 1.2%) of a grapefruit commercial phenolic extract. Addition of the preparation caused an increase of wet mass of ceacal digesta. Dry matter content decreased from 22% to 7.0% and pH increased from 7.11 to 8.0 in the control group and experimental groups, respectively. Increased pH was a consequence of the lower concentration of volatile fatty acids in rats fed the extract (77 μmol/g in control animals versus 8-13 μmol/g in extract treatments). However, glycolytic microbiological activity, measured indirectly through the measurement of its enzyme activity, decreased only in rats fed a diet containing the lowest level of the preparation.