Background: Insulin-like growth factor-I (IGF-I) is a peptide growth factor whose biological effects are mediated through a specific receptor (IGF-IR). IGF-I and IGF-IR are detected in both fetal and adult kidneys and have both metabolic and growth effects. IGF-IR expression during postnatal kidney development is not well defined and the biological role of this receptor during the postnatal stage is not clearly established. The purpose of the present study was to analyze IGF-IR gene expression during the postnatal development of rabbit kidney to achieve a better understanding of the correlation between growth and differentiation of kidney tissues and IGF-IR expression. Methods: Using in situ hybridization, we studied changes in IGF-IR expression in the kidneys of newborn rabbits and those up to 35 days old. Evaluation of the stage of kidney development and morphological maturation was made on histological sections stained with hematoxylin-eosin. Results: High levels of IGF-IR gene expression in the rabbit kidney occurred in the last stages of postnatal development and in the adult stages; during the development of the subcapsular metanephrogenic zone, IGF-IR gene expression was not observed. IGF-IR mRNA was expressed by proximal and distal tubules and by collecting ducts after these tissues attained morphological maturation. The appearance of IGF-IR mRNA in these kidney structures followed a precise temporo-spatial sequence. IGF-IR was not expressed by renal corpuscles, Henle's loops, inner medullary collecting ducts, vessels, or interstitial cells at any study stage. Conclusions: The temporal and spatial patterns of IGF-IR gene expression during postnatal development of the rabbit kidney suggest that IGF-IR and its ligands are relevant for the acquisition of the function, and not for development events, by proximal and distal tubules and collecting ducts. This study also suggests that IGF-IR mRNA localization constitutes a useful marker to determine the functional maturation of these renal structures.