A tetracycline-sensitive inducible expression system was used to regulate the expression of neurotransmitter receptor genes in two mammalian cell lines. The dopamine D3-receptor was stably expressed in GH3 cells, and GluR6 (a glutamate receptor subunit) was stably expressed in human embryonic kidney (HEK 293) cells. Three striking differences were found. 1) In the inactive state, virtually no D3-receptor expression was found in GH3 cells, whereas substantial levels of GluR6 expression were found in HEK 293 cells. 2) The induction of expression obtained upon removal of tetracycline was robust in GH3 cells but only modest in HEK 293 cells. 3) Whereas in each clonal cell line, the expression of a co-transfected hybrid transactivator is clearly regulated in a tetracycline-responsive manner, in the induced state, its mRNA levels were found to be very low in GH3 cells and very high in HEK 293 cells. The results indicate that, in contrast to GH3 cells, HEK 293 cells do not provide a cellular environment in which the expression of a heterologous gene can be tightly controlled in a tetracycline-responsive manner.