Angiogenesis plays a key role in solid tumor formation, invasiveness and metastasis. Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen that is necessary in the process of neovascularisation. Antagonists of growth hormone-releasing hormone (GH-RH) have been shown to suppress both in vivo and in vitro growth and metastasis of many human cancer cell lines. The mechanisms that mediate the antitumorigenic actions of these antagonists involve direct and indirect pathways, but are not completely elucidated. We have examined the effect of GH-RH antagonist MZ-4-71 on proliferation activity and VEGF release from cultured murine endothelial cells HECa10 in vitro. MZ-4-71 at 10(-8) to 10(-6) M concentrations inhibited the proliferative activity of cultured cells and suppressed the release of VEGF into supernatants of 72 h endothelial cell cultures. To our knowledge this is the first study reporting antiangiogenic properties of GH-RH antagonists.