Gastrin releasing peptide (GRP) is released by the postganglionic fibres of the vagus nerve which innervate the G cells of the stomach and stimulate them to release gastrin. GRP can directly stimulate pepsinogen release from chief cells by a specific GRP receptor that mobilizes intracellular calcium. Gastrin-releasing peptide has a prominent role as a tumour marker in the diagnosis of small-cell lung carcinoma.