Effects of body size on the kinetics of glucose metabolism were examined. In trial 1, steers were fed 6 kg of an 80:20 concentrate:roughage diet daily across a weight range of 226 to 470 kg. In trial 2, steers were fed 4.5 kg of a 45:55 concentrate:roughage diet daily across a weight range of 134 to 390 kg. Glucose irreversible loss averaged 820 +/- 32 and 461 +/- 13 mg/min for trials 1 and 2 and was not affected by body size. Thus, glucose irreversible loss seems to be regulated by feed intake when intake is above maintenance. Body size had no effect on glucose total entry rate, recycling or pool size. Blood glucose concentration and glucose space decreased at 0.004 mg X ml-1 X kg-1 and 0.002 fractional units body wt X kg-1 as body size increased. Digestible energy and nitrogen balance were measured in trial 2. Digestibility of energy increased at 0.0054 Mcal X d-1 X kg-1 as body size increased. Nitrogen retention averaged 28.2 g X d-1 and was not influenced by body size even though energy intake approached maintenance amounts at the end of the trial. On the basis of urinary nitrogen excretion data, we conclude that the maximum net contribution of amino acid carbon to glucose synthesis was 8% of glucose carbon.