Glycogen concentrations were measured by fluorescence enzymatic assay in rat brains fixed by focused microwave irradiation Regional glycogen concentrations varied 2.5 fold, ranging from 27.1 nmol glucosyl units/mg protein in the striatum to 52.3 nmol/mg protein in the cerebellum and 69.9 nmol/mg protein in the pons-medulla; other regions were intermediate in concentration. Since glycogen is contained primarily within astrocytes, this regional variation may reflect regional differences in astrocyte energy metabolism.