Comparison of the adult brain insulin receptor (IR) to other tissue IR demonstrates that the former migrates ∼10 kD faster on sodium dodecyl sulfate-polyacrylamide gel electrophoresis due to deficient sialic acid content of the asparagine N-linked carbohydrate moieties. We studied these receptors in the fetal rat (18-day) brain (∼125 kD) and liver (∼135 kD), and demonstrated that similar differences are present during fetal life. These differences are not modified by hyperglycemia associated with both mild hyperinsulinemia and normoinsulinemia/ hypoinsulinemia. We further studied the specific brain cell types: neurons, glial cells, and purified microvessel preparation, and demonstrated a heterogeneity in the N-linked glycosylation of the IR within an organ (brain). The neuronal (∼125 kD) and microvascular (∼125 kD, ∼135 kD) IR are deficient in sialic acid, thus conferring neuraminidase-insensitivity to the whole brain, whereas the glial cell IR, similar to the liver IR, exhibits neuraminidase sensitivity and migrates intermediate (∼128 kD) to the liver and brain IR. The functional significance of this receptor heterogeneity between various tissues and cells within the same organ (brain) remains to be determined.