To establish effective methods of enteral 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) administration, the efficiency of FDG absorption in the gastrointestinal tracts following enteral administrations was evaluated using the FDG biodistribution in normal rodents, in combination with various fasting conditions and FDG diluents. The blood FDG curve using hypotonic solution showed a rapid increase, while that in iso- and hypertonic groups showed slow rises. Brain FDG uptake had a close positive correlation with blood AUC (area under curve) and an inverse relationship with the stomach contents.