Two hypotheses were tested in gonad-intact, young (aged 6-18 months), growing red deer stags during an annual growth cycle. First, that glucose clearance rate is faster during summer than during winter. Secondly, that increased dietary protein availability will enhance winter growth. Stags were randomly assigned into one of two groups: group 1 (n = 5) had 16% while group 2 (n = 6) had 48% of dietary protein naturally protected against fermentative degradation in the rumen. Total crude protein and energy remained similar for each diet (12 and 14% respectively for protein and 11 MJ metabolisable energy/kg dry matter). Stags were kept indoors in individual pens for 12 months and given monthly intravenous glucose tolerance tests (IVGTT), at a dose of 200 mg/kg, in the fed and fasted (48 h) states to determine both growth and steady-state tissue requirements. Protein level had no effect on food intake, weight gain, insulin kinetics, or glucose clearance rate. In the fed state, insulin peak (highest level' after IVGTT) increased (P < 0.01) from October (139 pmol/l) to December (247 pmol/l) (S.E.D. = 42) and remained elevated during the summer, before declining (P < 0.01) from February (223 pmol/l) to April (130 pmol/l) (S.E.D. = 25). Glucose clearance rate was faster (P < 0.05) in December (1.69 litres/min) than June (0.61 litres/min) in the fed state (S.E.D. = 0.30), and decreased (P < 0.05) from February (1.75 litres/min) to April (0.92 litres/min) (S.E.D. = 0.39). During fasting, the pattern of glucose clearance was similar to that observed in the fed state, but the amplitude was lower, while the pattern for insulin peak was similar to that of the fed state. We concluded first, that additional protected protein does not benefit growth during winter. Secondly, we concluded from the fasted, steady-state data that stags are insulin resistant during summer. Thirdly, despite insulin resistance, data on the fed state demonstrated that stags have higher tissue energy requirements during summer growth.