Blood samples were withdrawn once weekly from the jugular vein of 10 karakul lambs (ca. 17 kg) for 12 weeks. The lambs were randomly allocated to a treatment and fed either a high-fibre (HF, n = 5) or low-fibre (LF, n = 5) diet. Dietary carbohydrate was provided as either structural (HF treatment) or readily fermentable (LF treatment) whereas the energy and nitrogen ratios were kept constant. Blood samples were drawn once a week and plasma concentrations of glucose, free fatty acids (FFA), lactate, insulin and glucagon were determined. Differences in the efficiency of metabolizable energy utilisation were determined from the average daily intake (ADI), average daily gain (ADG) and feed conversion ratio (FCR). ADI, ADG and FCR as well as the plasma composition remained constant within a treatment over the 12-week period. All data within each group of lambs were subsequently pooled. The ADI was higher in the HF than the LF (p ≤ 0.05), whereas the digestible energy and nitrogen intakes were similar for the two treatments. The plasma glucose and FFA concentrations were higher (p ≤ 0.05) in the LF than in the HF lambs. Insulin levels were higher (p ≤ 0.05) in the HF than in the LF lambs (1.39 and 1.66 ± 0.07 ng.ml-¹, respectively). There were no differences in the glucagon concentrations between groups. The insulin-glucagon ratio was, however, higher (p ≤ 0.05) in the HF than the LF group. The LF lambs maintained a higher ADG (p ≤ 0.05) than the HF lambs (160 and 115 ± 3.6 g.day–¹`, respectively) and at the same time had a lower (p ≤ 0.05) FCR than the HF lambs (5.9 and 10.1 ± 0.75, respectively). The LF lambs were more efficient (p ≤ 0.05) at converting dietary energy into body mass than the HF lambs, requiring 68.9 MJ ME per kg gain compared to 98.3 ¡À 5.48 MJ ME per kg gain of the HF lambs.