The dwarf little (lit) mouse is a model for the human hereditary disorder, isolated growth hormone (GH) deficiency type I. In these animals, dwarfism results from an autosomal recessively inherited gene mutation. The GH gene is present but production of GH mRNA is deficient, resulting in reduced serum GH and concomitantly decreased serum somatomedin. Growth retardation is evident by 15 days of age and adult animals reach approximately one-half normal size. Mutant mice of both sexes also exhibit a delayed onset of puberty, with males having a high degree of infertility. As administration of GH restores growth, we reasoned that growth failure in the mutant mice might be corrected by providing them with sufficient GH by gene therapy. Here we demonstrate that although the rat and human GH genes alone do not restore growth in transgenic mutants, a metallothionein-rat growth hormone fusion gene (MT-rGH) does. Moreover, the fertility of transgenic mutant males is improved; however, female fertility is impaired.