Human IGF-I was over-expressed in skeletal muscles of C57/BL6 × CBA mice under the control of the rat skeletal α-actin gene promoter. RT-PCR verified expression of the transgene in skeletal muscle but not in the liver of 1- and 21-day old heterozygote transgenic mice. The concentration of endogenous mouse IGF-I, measured by an immunoassay which does not detect human IGF-I, was not significantly different between transgenic mice and wild-type littermates (9.5 ± 0.8 and 13.3 ± 1.9 ng/g in muscle; 158.3 ± 18.6 and 132.9 ± 33.1 ng/ml in plasma, respectively). In contrast, quantitation with antibodies to human IGF-I showed an increase in IGF-I of about 100 ng/ml in plasma and 150 ng/g in muscle of transgenic mice at 6 months of age. Transgenic males, compared to their age matched wild-type littermates, had a significantly higher body weight (38.6 ± 0.53 g vs. 35.8 ± 0.64 g at 6 months of age; P < 0.001), dry fat-free carcass mass (5.51 ± 0.085 vs. 5.08 ± 0.092 g; P < 0.001) and myofibrillar protein mass (1.62 ± 0.045 vs. 1.49 ± 0.048 g; P < 0.05), although the fractional content of fat in the carcass was lower (167 ± 7.0 vs. 197 ± 7.7 g/kg wet weight) in transgenic animals. There was no evidence of muscle hypertrophy and no change in the proportion of slow type I myofibres in the limb muscles of Rskα-actin/hIGF-I transgenic mice at 3 or 6 months of age. Phenotypic changes in Rskα-actin/hIGF-I mice are likely to be due to systemic as well as autocrine/paracrine effects of overproduction of IGF-I due to expression of the human IGF-I transgene. The effect of muscle specific over-expression of Rskα-actin/hIGF-I transgene was tested on: (i) muscle regeneration in auto-transplanted whole muscle grafts; (ii) myofibre atrophy following sciatic nerve transection; and (iii) sarolemmal damage and myofibre necrosis in dystrophic mdx muscle. No beneficial effect of muscle specific over-expression of Rskα-actin/hIGF-I transgene was seen in these three experimental models.