Estimates of the glucose pool, the glucose space, the turnover rate, and the recycling of glucose were made after the injection of [U-14C]glucose into (a) obese rats fed a high fat diet and (b) rats fed a carbohydrate diet. The specific activity--time curve consisted of two components. Physiological parameters were calculated by using a two-compartment model. The glucose pool and glucose space were the same in both groups of rats. The turnover rate was 1.96 mg. min-1 for the carbohydrate-fed rats and 1.55 mg. min-1 for the fat-fed rats. There was about 12 percent recycling in both groups. In the carbohydrate-fed group, another approach based on simultaneous use of [6-14C]glucose and [6-3H]glucose yielded nearly the same values for these parameters. Respiratory excretion of CO2 and the incorporation of labeled glucose into lipids of some tissues were also measured. The rate of excretion of labeled CO2 and the conversion of labeled glucose into fatty acids in fat-fed rats were lower than in the carbohydrate-fed rats by 50 percent and 80 percent, respectively. More glucose was diverted into glyceride glycerol in the fat-fed group. It is suggested on the basis of the results that glyceride glycerol can serve as a gluconeogenic substrate in these rats where the turnover rate of glucose is much higher than the daily intake of carbohydrates.