The rate of appearance of unlabelled glucose was calculated from tracer data and compared with the actual rate of infusion of unlabelled glucose into a anaesthetized dog with all sources of endogenous glucose production surgically removed. The mean steady-state rate of appearance of unlabelled glucose calculated from the equilibrium specific radioactivity was insignificantly higher (0.3%) than the actual rate of infusion of unlabelled glucose (n = 6). During non-steady states, a time-variable volume of distribution of glucose (V) was necessary to predict the rate of appearance of unlabelled glucose correctly from the pool-dependent equation described by Steele [(1959) Ann. N.Y. Acad. Sci. 82, 420–430]. Rapid fluctuations in the rate of appearance of glucose could be predicted reasonably well by using a fixed value of V for 40ml/kg, but by using larger fixed values for V (100–160ml/kg) the rates were inaccurate. The pool-dependent two-radiactive-isotope technique described by Issekutz, Issekutz & Elahi [(1974) Can. J. Physiol. Pharmacol. 52, 215–224] predicted single-step increases in the rate of infusion of glucose reasonably accurately, but the Steele (1959) equation was better at predicting sequential changes in the rate of infusion of unlabelled glucose.

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