Transgenic mice harbouring mouse metallothionein I-human growth hormone (MT-hGH) fusion genes were produced using the microinjection technique. The bones of adult MT-hGH transgenic mice, which continuously expressed high levels of hGH in their serum, and age-matched controls lacking detectable concentrations of hGH were measured microscopically. In addition to analyzing absolute skeletal dimensions, measurements were related to the cube root of the maximum body weight of the same animal. Absolute values obtained from transgenic mice were significantly higher than those obtained from controls for most of the defined measurements. However, the increase in skeletal dimensions was mostly not as pronounced as the increase in body weight and all bones were not affected to the same extent. There was no significant correlation between the serum GH concentration in individual mice and their degree of bony overgrowth. A disproportionate skeletal gigantism in MT-hGH transgenic mice may result from time differences in epiphyseal union of various bones of both sexes as well as differences in mechanical bone loading due to a drastically increased body weight. Individual concentrations of locally produced tissue insulin-like growth factor I (IGF I) might also play a role. Possible effects of these factors are discussed. The results presented in this study show that MT-hGH transgenic mice provide a powerful tool for the investigation of hormonal regulation of bone growth.