Bombesin (BN) or gastrin-releasing peptide (GRP) can stimulate the growth of neoplasms such as breast cancer and small-cell lung carcinoma (SCLC). Antagonists of BN/GRP have been shown to inhibit these cancers. We evaluated whether antagonists of BN/GRP can suppress the growth of human non-SCLC (NSCLC) xenografted into nude mice. The effect of the administration of BN/GRP antagonist RC-3940-II on the growth of H460 and A549 NSCLC cell lines orthotopically xenografted into the intrapulmonary interstitium was examined. Protein levels of K-Ras, COX-2, Akt/pAkt, WT p53, Erk1/2, and lung resistance-related protein (LRP) in tumors were analyzed by Western blot analaysis, and receptors for BN/GRP were investigated by radioligand-binding studies. The effect of RC-3940-II on the proliferation of H460 and A549 cells in vitro was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays. High-affinity receptors for BN/GRP were found on tumors. Treatment with RC-3940-II significantly (P < 0.001) inhibited growth of H460 and A549 NSCLC xenografts by 30-50% and led to an improved performance status, compared with controls. In H460 NSCLC, the antitumor effect was associated with a significant (P < 0.001) reduction in protein levels of K-Ras, COX-2, pAkt, and pERK1/2 and with a major augmentation in the expression of WT p53, compared with controls. In A549 NSCLC, pAkt and LRP were significantly down-regulated. Our findings demonstrate the efficacy of BN/GRP antagonist RC-3940-II for the treatment of NSCLC. The suppression of K-Ras, COX-2, pAkt, and LRP, as well as the up-regulation of WT p53 might contribute to the antitumor action of BN/GRP antagonists.