Rhubarb (Rheum rhapontiam) stalk fibre was previously shown to be hypolipidaemic under clinical and experimental conditions. The present study was undertaken to investigate whether rhubarb stalk fibre has a hypolipidaemic effect under diabetic conditions. Two models of diabetic rats were used: streptozotocin-induced diabetic rats, and diabetes-prone BB (BBdp) rats. The plasma cholesterol and triacylglycerol concentrations were elevated after the onset of diabetes in BBdp rats, but not in sterptozotocin-induced diabetic rats. The rhubarb-fibre diet had no effect on the plasma cholesterol or triacylglycerol concentrations of diabetic rats. The hypolipidaemic effect of rhubarb stalk fibre has been suggested to be due to the bile-acid-binding capacity of rhubarb fibre, which in turn up regulates cholesterol 7alpha-hydroxylase (cyp7a) activity. cyp7a is the first and the rate-limiting enzyme in the breakdown of cholesterol to bile acids. We measured the cyp7a activity and mRNA levels in control and diabetic rats fed rhubarb- and cellulose-fibre diets. The cyp7a activity and mRNA abundance were increased in both diabetic rat models, indicating that bile acid synthesis is enhanced in diabetes. Feeding a diet enriched with rhubarb fibre caused a slight but significant increase (P<0.05) in cyp7a enzyme activity in BBdp rats, but no change in cyp7a mRNA abundance was detected. These results suggest that although a rhubarb-fibre-enriched diet increased cyp7a activity in BBdp rats, there was no apparent therapeutic benefit in terms of lowering plasma cholesterol concentrations.