Corticotropin releasing hormone is a neurotransmitter in the inferior olive complex of marsupials and mammals. The ontogeny of corticotropin releasing hormone gene expression in the rat inferior olive has not been described. Using in-situ hybridization histochemistry in 25 animals, we established the developmental profile of the peptide's messenger ribonucleic acid in the postnatal rat. CRH-messenger RNA was first detectable in two of four animals on the fifth postnatal day. Subsequently, gene expression increased linearly with age: by day 14, CRH was expressed in all olivary complex nuclei, and the distribution and relative abundance on day 18 were comparable to those in the adult. The developmental profile of CRH-mRNA in the rat inferior olive differs from those in the mouse and opossum, and from the pattern in the rat hypothalamus, suggesting species- and site-specificity of the peptide's function.