Cell surface expression of the NR1a subunit has been examined in mouse L cell lines permanently transfected with the complementary DNA for human NR1a or with the complementary DNAs for NR1a and NR2A. The expression of the subunits was under the control of the murine mammary tumour virus promoter and following induction of expression by dexamethazone both cell lines expressed high levels of the NR1a subunit as determined by immunofluorescence using permeabilized cells and immunoblotting of cell membranes with subunit specific antibodies. However, cell surface expression of the NR1a subunit was found only in the cells expressing both the NR1a and NR2A subunits. This was confirmed by cell surface biotinylation of the two cell lines and affinity isolation of the receptor subunits. To determine if this result was solely due to the use of a particular cell line and or the choice of expression vector, Cos-7 cells were transiently transfected with either NR1a or NR1a plus NR2A. Here too cell surface expression was only found following co-transfection of both subunits.