Several genetic mutations in mice and rats that produce lifelong growth hormone (GH) deficiency result in overexpression of GH-releasing hormone (GHRH) mRNA in hypothalamic arcuate nucleus neurons. In order to examine the development of this condition, GHRH mRNA expression was quantified in Ames dwarf (df/df) and normal (DF/?) mice at 1 (day of birth), 3, 7, 14, 21 and 60 postnatal days (d) following in situ hybridization. Total mRNA was assessed using computer-assisted densitometry after X-ray film autoradiography, and mRNA expression per neuron was quantified by counts of grains per cell after emulsion autoradiography. Total GHRH mRNA was the same in dwarf and normal mice at 1, 3 and 7d. GHRH mRNA in dwarfs increased at 14d to 240% of that in DF/? (p < 0.005); the percentage overexpression in dwarf mice remained ≧200% through 60d, although total GHRH mRNA increased in both dwarfs and normals during this period. GHRH mRNA per neuron was the same in normal and dwarf mice at 1d, then increased in dwarfs to 190% of that in normals at 3d (p < 0.05), and rose to 300% of normal levels by 7d and beyond (p < 0.005). There was no sexual dimorphism in expression by either measure in normal or dwarf mice. These results indicate that an increase in GHRH mRNA in Ames dwarf mice is first detectable at 3d, a period of approximately 7d after the failure to initiate GH production, which occurs normally at embryonic day 17.5. The onset of GHRH overexpression occurs earlier than the decline of either hypophysiotropic somatostatin or dopamine in Ames dwarf mice. This difference may be due to the stimulatory action of GHRH, as opposed to the inhibitory effects of factors examined previously.