Growth hormone and mechanical loading are known to be important factors influencing bone growth. We have measured proportions of the skull and the postcranial skeleton of metallothionein-human growth hormone (MT-hGH) transgenic mice expressing high levels of hGH in their serum, of NMRI mice being large as a result of selection for high 8-week body weight (N8), and of unselected controls (Pop) derived from the NMRI strain. Absolute bony dimensions of transgenic mice were as a rule significantly larger than those of controls, the differences ranging between 3% and 32% in males and from 6% to 28% in females. By contrast, the enlargement of skeletal dimensions of N8 mice did not exceed 10% and was restricted to distinct bones. When related to the cube root of maximum body weight of the same animal, bones of controls were as a rule larger than those of N8 and MT-hGH transgenic mice. A detailed analysis of bony dimensions of GH transgenic mice and of mice selected for high body weight was carried out to judge the effects of GH overexpression and mechanical loading due to increased body weight on bone growth. The fact that bones of transgenics were as a rule larger than those of selected mice in spite of both groups reaching similar maximum body weights, suggests that skeletal gigantism in MT-hGH transgenic mice can only in part be a result of increased body weight.