Purpose This study seeks to determine if growth hormonereleasing hormone (GHRH) antagonist, JMR-132, increases survival when given before whole body radiation. Material and methods C3H mice were divided into 14 groups. The first 7 groups were given whole body radiation alone in the increasing doses of 7, 7.5, 8, 8.5, 9, 10, and 11 Gy, respectively. The other 7 groups received JMR-132 (10 μg/day s.c.) for 3 weeks then received the whole body radiation in increasing doses of 7, 7.5, 8, 8.5, 9, 10, and 11 Gy, respectively. The experiment lasted 35 days. Hazard ratios for survival were calculated for the addition of the GHRH antagonist as compared to radiation alone at the different dose levels. RT-PCR was performed to identify potential target genes. CD31 immunohistochemical staining identified the differences between average vessel count. Results Mice pretreated with JMR-132 showed a longer survival at lower radiation doses, the hazard ratios were 0.34 and 0.59 when the drug was given with doses of 7.5 and 8 Gy, respectively. A statistically significant higher hazard ratio of 3.48 and 4.22 was seen in mice when GHRH antagonist was added to doses of 10 and 11 Gy, respectively. Organ specific upregulation of several target genes such as Akt2, ATM, and Trp53 was seen with the GHRH antagonist. In the animals pretreated with JMR-132 the small intestine and the kidney showed higher average vessel count. Conclusion Pretreatment with GHRH antagonist JMR-132 protects at lower doses of radiation.