Antagonists of growth hormone-releasing hormone (GH-RH) inhibit the growth of various tumors through mechanisms that involve the suppression of the insulin-like growth factor I and/or insulin-like growth factor II levels or secretion. In the present study, we tested the hypothesis that the tumor inhibition is associated with a decrease in telomerase activity because telomerase is considered obligatory for continued tumor growth. Nude mice bearing xenografts of U-87MG human glioblastomas were treated with GH-RH antagonist MZ-5-156. Telomerase activity was assessed by the telomerase repeat amplification protocol. Treatment with MZ-5-156 reduced levels of telomerase activity as compared with controls. When U-87 glioblastomas, H-69 small cell lung carcinomas, H-23 non-small cell lung carcinomas, and MDA-MB-468 breast carcinoma cells were cultured in vitro, addition of 3 microM MZ-5-156 also inhibited telomerase activity. Reverse transcription-PCR analysis revealed that in U-87MG glioblastomas, the expression of the hTRT gene encoding for the telomerase catalytic subunit was significantly decreased by MZ-5-156, whereas the levels of mRNA for hTR and TP1, which encode for the telomerase RNA and telomerase-associated protein, respectively, were unaffected. The repression of the telomerase activity was not accompanied by a significant decrease of mRNA level for the c-myc protooncogene that regulates telomerase. Our findings suggest that tumor inhibition induced by the GH-RH antagonists in U-87MG glioblastomas is associated with the down-regulation of the hTRT gene, resulting in a decrease in telomerase activity. Further studies are needed to establish whether GH-RH antagonists produce telomerase inhibition in other tumors.