The objectives and the methodology of mammalian gene transfer are discussed and findings in growth hormone (GH) transgenic mice are reported to illustrate the potential offered by genetically designed animal models for investigations in various areas of biomedical research. Transgenic mice expressing hybrid genes composed of either human or bovine GH coding sequences fused to the mouse metallothionein I promoter show high serum levels of heterologous GH, increased growth rates and final adult size, decreased life expectancy and a variety of pathological changes.