Under strictly basal conditions (fasting and resting state) a free glucose space can be measured by the analysis of the rapid intravenous glucose tolerance test. Under these conditions, excluding the interference of factors which might modify the cellular permeability to glucose, the clear cut difference between the speed of diffusion and that of cellular penetration allows of a calculation of a volume of glucose diffusion. The results obtained by this method are comparable to those of other estimations of the extracellular compartment.