Body weight gain and external body dimensions of MT-hGH transgenic mice were compared with mice (NMRI) selected for high 8-week body weight (N8) and unselected controls derived from the NMRI strain (Pop). The growth curves from day 30 to 120 of transgenic mice exhibited a significantly steeper slope than those of male and female controls and of female N8 mice and did not show sex-related differences. The continuous pattern of GH secretion in transgenic mice is discussed as a possible reason for this phenomenon. Body weight gain of transgenic mice did not significantly exceed that of male N8 mice. None of the groups showed an obvious prolongation of the period of rapid daily weight gain. Maximum body weights of male and female transgenic mice were significantly higher than those of sex-matched controls but not of N8 mice. A drastic loss of body weight of about 25% of the maximum value was observed in the transgenic group prior to death. External body dimensions were largest in MT-hGH transgenic animals, followed by N8 mice and controls. In addition to these absolute measurements, values were related to the cube root of maximum body weight of the same animal. This is the first study that provides a comparative analysis of the effects of GH gene transfer and selection for body weight gain on body growth of mice derived from an outbred strain.