Abstract Rats were intravenously injected with 10μCi of [U-14C]deoxyglucose (DG) or [U-14C]fluorodeoxyglucose (FDG) and sacrificed by microwave irradiation 4, 45 and 240 min later. Fluorodeoxyglucose phosphate (FDGP) accumulated at a significantly greater rate than did deoxyglucose phosphate (DGP) in brain. Loss of the phosphorylated compounds from brain between 45 and 240 min after administration was similar. The per cent of radioactivity in non-phosphorylated compounds was lower with FDG as tracer at all times after injection. The probable basis for the difference in rate of phosphorylation of the two compounds is a difference in the kinetic properties of rat brain hexokinase with FDG and DG as substrates. The principal use of these isotopes is for studies of regional glucose utilization in brain. In the rat, our data indicate that FDG has two advantages over DG for such studies. Since FDGP accumulates in brain at about 150% the rate of DGP, the amounts (and costs) of isotope can be reduced by up to one third with FDG as tracer. The more rapid decrease in background of non-phosphorylated FDG potentially allows the study of shorter periods of time in autoradiographic work. These considerations apply to both qualitative and quantitative studies of glucose utilization by rat brain. For quantitative work, however, the constants necessary to convert rates of FDG phosphorylation to rates of glucose phosphorylation by rat brain have yet to be determined.