1. Conventional and germ-free rats were fed a fibre-free elemental diet with or without the addition of fermentable dietary fibres. We have previously reported that fibre was associated with greatly increased epithelial cell proliferation, but only in the conventional group, implying that it is the breakdown of fibre by the colonic microflora that is the main determinant of mucosal proliferation in the hind gut. The relationship of these changes to various plasma hormones implicated in intestinal growth control are described in this paper. 2. The most dramatic finding was that plasma levels of enteroglucagon and peptide YY were greatly increased in the germ-free groups. The response of these rats to fibre differed in that fibre decreased levels of enteroglucagon and peptide YY in the germ-free animals, but increased them in the conventional rats. Gastrin and insulin levels were significantly lowered in the fibre-supplemented groups, but were not affected by the microflora. 3. These results corroborate our previous findings that the effects of fibre and its fermentation are dynamically complex, and demonstrate that, like proliferation, direct effects and indirect fermentation-derived effects on plasma hormones also coexist.