During the 3rd wk of postnatal life in the rat, dramatic maturational changes occur in the structure and function of the small intestine, enabling the animal to make the transition from milk to solid food. To investigate the role of GH in the regulation of this complex process, we studied postnatal intestinal maturation in the spontaneous dwarf rat, a strain of Sprague-Dawley rats with an autosomal recessive mutation in the GH gene resulting in complete but isolated GH deficiency. GH-deficient and GH-normal littermates were studied at d 7 and 14 (suckling) and d 23 (postweaned). The body weight of GH-deficient animals was inhibited by 60% at each age. Longitudinal growth of the small intestine was not inhibited, suggesting that longitudinal small bowel growth is independent of GH regulation. Mucosal cell mass was significantly lower in GH deficiency at all ages studied, and digestive hydrolase capacity per cm of intestine was significantly lower in GH-deficient postweaned animals. However, epithelial cell mass increased markedly in association with weaning and the maturation of lactase, sucrase, and aminooligopeptidase proceeded normally in GH deficiency. These data suggest that, although GH is not required for normal postnatal intestinal maturation, the mucosal epithelial hypoplasia found in GH-deficient animals suggests that GH or GH-dependent factors act as an intestinal mucosal growth factor whose function is to promote the homeostatic or steady-state regulation of mucosal epithelial growth.