OBJECTIVE Our objective was to characterize the local vascular effects of gastrin-releasing peptide in forearm resistance vessels as a biomarker to aid the clinical development of broad-spectrum neuropeptide antagonists as anticancer agents. METHODS On different occasions, 7 healthy male volunteers received brachial artery infusions of gastrin-releasing peptide, and forearm blood flow was measured by venous occlusion plethysmography. Dose-finding studies identified a range of pharmacologically active doses of gastrin-releasing peptide (3 to 450 pmol/min) for use in dose response and tachyphylaxis studies. The nitric oxide clamp method was used for the assessment of the contribution of nitric oxide to the vasodilatory effect shown. CONCLUSIONS Gastrin-releasing peptide proved to be a potent, dose-dependent arteriolar vasodilator. The time to onset and offset of effect was rapid, within minutes, and marked tachyphylaxis was shown, with recovery by approximately 20 minutes. The contribution of nitric oxide to this vasodilatation was minimal. Forearm blood flow studies with the use of gastrin-releasing peptide may be usefully incorporated into the further clinical development of broad-spectrum neuropeptide growth factor antagonists.