The inhibition of growth of various hormone-dependent cancers by analogs of luteinizing hormone-releasing hormone (LH-RH) may be exerted in part through receptors for LH-RH present on tumor cells, but the direct mode of action of LH-RH agonists and antagonists is still not completely understood. The aim of this study was to investigate the effects of agonist [D-Trp6]LH-RH and antagonist Cetrorelix, administered s.c. at a dose of 100 microg/day for 3 weeks on the binding characteristics and subcellular localization of receptors for LH-RH in OV-1063 human epithelial ovarian cancers xenografted into nude mice. Using radioligand binding studies, following in vitro desaturation, we demonstrated the presence of specific, high affinity binding sites for LH-RH in both cell membrane and nuclear fraction of OV-1063 tumors. Treatment with Cetrorelix, but not [D-Trp6]LH-RH, caused about 60% reduction (p<0. 01) in tumor volume and weight. [D-Trp6]LH-RH decreased the number of LH-RH receptors on OV-1063 tumor membranes by 44% after 14 days (p<0.01), and the concentration of receptors remained at that level on day 21. The maximal binding capacity of receptors for LH-RH in the nuclei was significantly higher (p<0.05) after 3 weeks of treatment with [D-Trp6]LH-RH. Cetrorelix decreased the concentration of membrane receptors for LH-RH by 53% (p<0.01) after 14 days and the levels on day 21 were even lower, showing a 70% reduction (p<0. 01). In contrast, the number of LH-RH binding sites in the nuclear pellet was significantly increased (p<0.01) by Cetrorelix at that time. Our results demonstrate for the first time that the down-regulation of LH-RH receptors on the cell membranes of OV-1063 human ovarian cancers after therapy with antagonist Cetrorelix or agonist [D-Trp6]LH-RH is associated with an increase in receptor concentration in the nuclei. These phenomena could be related to the internalization and subcellular translocation of receptors in these tumor cells.