Ten Dohne merino wethers with an average mass of 50 kg, were used to study differencesin ruminal VFA production rates, whole body kinetics of VFA, glucose and FFA as well as blood hormone concentrations when fed a high- or a low-fibre diet. Dietary carbohydrate was provided as either structural (HF treatment) or readily fermentable (LF treatment), while keeping energy and nitrogen intakes constant. The wethers were fitted with ruminal cannula and with an indwelling catheter in the abdominal aorta and a temporary catheter inserted into the jugular vein. Either (1-14c)-acetate or (2-14c)-propionate as continuously infused into the rumen concurrently with either a (9,10 N-3H)FFA or(6-3H)-glucose, respectively, infusion into the jugular vein. Blood and digesta samples were drawn simultaneously after plateau specific radioactivity in all labelled markers had been achieved. Total ruminal VFA, acetate and butyrate production rates were not altered by diet, whereas propionate production rates were higher (p < 0.05) for the LF than the HF treatment, 2.86 and 2.45 Â± 0.04 mol.day-l , respectively. Arterial propionate concentrations were lower (p< 0.05) in the LF than the HF treatment (0.121 and 0.164 Â± 0.005 mM, respectively) whereas the glucose concentrations were higher in the LF than the HF treatment (4.39, and 3.61 Â± 0.177 mM, respectively). There were no differences in the arterial concentrations of acetate and propionate between the two treatments. The glucose irreversible loss rate was higher (p < 0.05) in the LF than the HF treatment (42.2 and 36.1 Â± 1.98 mmol.h-1, respectively), whereas the FFA entry rate was lower in the LF than the HF treatment (6.2 and 10.5 Â± 0.541 mmo1.h-1 , respectively). Insulin, glucagon and thyroxin concentrations were higher (p < 0.05) in the HF than the LF treatment. Triiodothyronine concentrations were similar for both treatments. The arterial oxygen and carbon dioxide concentrations were higher (p < 0.05) in the HF than the LF treatment (HF: 5.07 and 18.9 mM, respectively; LF: 4.58 Â± 0.22 and 16.0 Â± 0.875 mM, respectively).