At least two different models for the transmembrane topology of the glutamate receptor subunits have been proposed. We investigated some features of these two models for the GluR1 subunit by inserting epitope tags between residues Lys(502)-Pro(503), Ala(632)-Glu(633), Lys(712)-Pro(713) or after the C-terminal residue Leu(889). The accessibility of the tags then was detected using a tag-specific antibody before and after detergent-permeabilizing oocytes expressing the tagged subunits. The epitope tag inserted between residues Lys(712)-Pro(713) is extracellular and after Leu(889) intracellular, Epitope tags inserted between residues Lys(502)-Pro(503) and residues Ala(632)-Glu(633) were not detectable. Collectively, these results provide supporting evidence for a previously proposed topological model of GluR subunits containing an N-terminal extracellular domain, three transmembrane domains, the first two of which are bridged by a reentrant membrane pore-lining loop, and an intracellular C-terminal domain.