The aim of the study was to compare the physiological consequences of two dietary fibre sources on the faecal microflora and colonic mucosal growth in rats. The studied sources, a moderately well-soluble fibre (rice bran, RB) and a less well-soluble fibre (wheat bran, WB), were included in diets of rats at a level of 10% for 3 weeks and compared with a totally fibre-deprived diet. RB significantly increased faecal water compared to the control diet (p < 0.05). Faecal nitrogen content and bacterial mass, as estimated from the 2,6-diaminopimelic acid (DAPA) output, were greatly and significantly increased by RB, and to a lesser extent by WB, compared to the control diet. Total bile acid excretion was significantly higher by rats fed RB than by those fed WB. Faecal bacterial enzyme activities tested (beta-glucuronidase, mucinase and nitroreductase) were significantly reduced by the two different fibre sources, but RB was more effective than WB, except for nitroreductase activity which was reduced at the same level for each fibre source. Although measurements of mucosal colonic weight and RNA content were significantly different between groups fed RB and WB (p < 0.05), DNA content and the ratio RNA/DNA did not significantly differ between these groups. Our results indicate that the differential changes observed in beta-glucuronidase and mucinase activities and DAPA and bile acid excretion may depend on the nature of the fibre consumed. They also suggest that RB, which had similar effects, sometimes more marked than WB, on the studied parameters, may be a new valuable fibre source.