We previously suggested that gonadotropin-releasing-hormone (GnRH) analogues activate the phosphoinositide pathway in rat mammary tumor membranes. In the present study we analyzed the binding of GnRH analogues to these membranes and assessed its modulation by guanine nucleotides. [125I]Buserelin (a GnRH superagonist) binding is specific because it is displaced only by GnRH analogues. Scatchard plot analysis reveals high affinity binding sites (Kd = 2.5 ± 0.8 nM, Bmax = 250 ± 120 fmol/mg membrane protein) and low affinity binding sites (Kd 1.1 ± 0.3 μM, Bmax = 200 ± 105 pmol/mg membrane protein). Guanine nucleotides increased the ED50 of [125I]buserelin displacement, and almost completely eliminated the high affinity binding. Similar results were obtained with [125I]d-Trp6-GnRH —another GnRH superagonist. The inhibition of buserelin binding by guanine nucleotides was specific for nucleotides that interact with G-binding proteins and was dose-dependent with a maximal effect at 10 μM GTPγS. Kinetic analysis of buserelin binding revealed that the dissociation rate increased at least 4-fold in the presence of 10 μM GTPγS. These results support the hypothesis that GnRH analogues interact directly with mammary tumors and activate a G-protein-dependent transducing mechanism.