GnRH receptors (GnRH-Rs) mediate direct antiproliferative effects on hormone-dependent cancer cells. GnRH-Rs can be grouped according to ligand specificity (for GnRH-I and -II), and there is evidence that type II GnRH ligands and/or receptors can inhibit proliferation. Type I GnRH-Rs (e.g. human and sheep) lack the C-terminal tails found in other G protein-coupled receptors including type II GnRH-Rs (e.g. Xenopus; XGnRH-R). This underlies the remarkable resistance of type I GnRH-Rs to desensitization and may be important for chronic effects on proliferation. To test this, we have compared the antiproliferative effects of GnRH-Rs expressed in MCF7 breast cancer cells using recombinant adenovirus (Ad). Endogenous GnRH-Rs were not detected, but infection with Ad-expressing sheep GnRH-Rs (sGnRH-R) facilitated proliferation inhibition by Buserelin, and maximum inhibition required only 10,000-20,000 sGnRH-Rs. XGnRH-Rs were much less efficient at inhibiting proliferation and were internalized faster than sGnRH-Rs. Thus, the type II GnRH-R is less efficient at inhibiting proliferation, presumably because it is rapidly desensitized and/or internalized. Moreover, comparisons of human GnRH-R, sGnRH-R, and XGnRH-R, as well as chimeric receptors (type I GnRH-Rs with C-terminal tails from XGnRH-Rs), revealed that C-terminal tail addition increases receptor expression and thereby increases the efficiency with which the vector facilitates the antiproliferative effect.