Sixteen prepubertal Friesian heifers were used to examine the effect of bovine growth hormone (GH) and ovariectomy (OVX) at 2.5 mo of age (2 x 2 factorial design) on growth, carcass quality, and fiber types, capillarization, and metabolic potentials of the longissimus muscle, and serum concentrations of estradiol-17beta (E2beta), insulin, GH, IGF-I, and IGF-binding proteins (IGFBP). Treatment with GH (15 mg/d) started at 147 +/- 3 kg BW and lasted for 15 wk. Heifers were fed a mixed roughage-based diet. Growth hormone increased ADG (P < .001), improved gain:feed (P < .007), and had a small but positive influence on lean accretion. Growth hormone reduced fat thickness (P < .009), carcass fat trim (P < .009) and i.m. fat (P < .09). Ovariectomy did not affect performance but increased dressing percentage (P < .03), full rib weight (P < .003), and fat thickness (P < .04). Ovariectomy reduced E2beta (P < .001) and insulin (P < .02), and increased the 32-kDa IGFBP (IGFBP-2) (P < .09). Growth hormone treatment increased GH, IGF-I, the 28-kDa IGFBP, and the 40- to 43-kDa IGFBP (IGFBP-3) (P < .004 or P < .001). Neither GH nor ovariectomy affected the proportion and relative area of the individual muscle fiber types, but GH tended to increase type I fiber area (P < .10). Number of capillaries per fiber increased in OVX GH-treated heifers (GH x OVX interaction, P < .02). Activities of citrate synthetase were higher in GH-treated (P < .05) and OVX (P < .02) heifers, indicating increased oxidative capacity of the longissimus muscle. The effects of GH on performance and carcass fattening were in accordance with the observed hormonal changes. When slaughter occurs before puberty, ovariectomy has no effect on performance, only few effects on carcass quality, and small effects on hormone concentrations.