Gastrin-releasing peptide (GRP), the mammalian homolog of bombesin, is often studied as a prototypic neuroregulatory hormone and growth factor, but its own regulation and physiological roles remain to be fully defined. We now demonstrate that the GRP gene is expressed in human thyroidal calcitonin (CT)-containing neuroendocrine cells (C-cells) in an ontogenic pattern similar to its expression in pulmonary neuroendocrine cells and is also expressed at high levels in C-cell hyperplasias and neoplasias (medullary carcinomas of the thyroid). Mean GRP-like immunoreactivity is 20 times higher in 3-week-old to 5-month-old infants than in normal adults, with six of seven infants having GRP levels 6- to 67-fold higher than those in normal adults, the highest levels occurring at 2-2.5 months. CT levels are about 100 times greater than GRP levels at all time intervals, with levels of GRP and CT being linearly correlated (r = 0.98). By RNA blot analysis, GRP mRNAs are increased in neonatal thyroids compared to adult thyroids. In situ hybridization and immunoperoxidase analyses localize GRP mRNAs and peptide to a majority of C-cells in fetuses and neonates, but to only 5-18% of C-cells in normal adults. The majority of developing C-cells have a dendritic morphology, suggesting a paracrine role, although this morphology is not observed in adult C-cells. In addition, for unknown reasons, an increased percentage of C-cells positive for GRP occurs in normal thyroid adjacent to GRP-negative follicular adenomas and papillary carcinomas, an association that we term perineoplastic. We hypothesize that GRP gene expression may play a role in both normal and neoplastic growth processes.