Twenty Dohne merino wethers with an average mass of 52 kg, were used to study the effect of varying the fibre content of the diet on differences in ruminal concentrations of volatile fatty acids (VFA) and the partial digestion of these diets. Dietary carbohydrate was provided as either structural (HF treatment) or readily fermentable (LF treatment), while energy and nitrogen intakes remained constant. The wethers were fitted with simple â€˜Tâ€™ cannulae in the rumen, abomasums and ileum. The diets were fed at half-hourly intervals. 51Cr-EDTA and 103RuP were infused intraruminally for 14 days and spot samples were withdrawn from the various cannulae over the last four days, in order to determine digesta flow and apparent digestion of the diets in the different gastrointestinal compartments. Changing the form of dietary carbohydrate had no effect on organic matter (OM) or nitrogen (N) digestion. Quantitatively more starch (p ≤ 0.05) was digested in the rumen (186 Â± 24.6g /day-1) and small intestines (35.6 Â± i2.6g /day-1) of the LF than the HF wethers, whereas more fibre (p ≤ 0.05) was digested in the rumen (84.2 Â± 10g /day-1) of the HF than the LF wethers. Ruminal fermentation was significantly affected by altering the fibre : concentrate ratio in the diet. The molar proportions of the VFA differed significantly (p ≤ 0.05) between the two treatments. The ruminal proportions of acetate, propionate and butyrate (mmol.mol-1) were o.721, 0.208 and 0.071 Â± 0.01, respectively, in the HF group and 0.645m 0.226 and 0.112 Â± 0.01, respectively, in the LF group. As a result the acetate : propionate ratio was lower (p ≤ 0.05) in the LF compared to the HF wethers (2.86 and 3.48 Â± 0.07, respectively). The main differences between the two treatments appeared to be an increased supply of glucogenic precursors (viz. glucose and propionate) to the host when the LF rather than the HF diet was fed.