Testosterone is known to act differentially on skeletal muscle from different regions. Two genes likely to mediate the testosterone effect are insulin-like growth factor-I (IGF-I), an important growth regulator acting in an autocrine and paracrine way, and androgen receptor (AR), as receptor density could account for differential muscle growth. Another muscle-specific gene that may play a role in differential muscle growth is myostatin, a member of the transforming growth factor-beta superfamily, shown to be a negative regulator of skeletal muscle mass. The objective of this study was to quantify and compare the expression of these three genes in two different skeletal muscles in sheep. East Friesian x Dorset sired ram lambs born from Dorset ewes were used in a 2 x 4 factorial experiment. Eighteen sets of twins were assigned to four age groups corresponding to 77, 105, 133 and 161 days of age and one individual from each set was castrated at birth. Total RNA was extracted from samples of semitendinosus (ST) and splenius (SP) muscles collected at the time of slaughter. Insulin-like growth factor I mRNA was measured using competitive reverse-transcription-polymerase chain reaction (RT-PCR). Androgen receptor and MSTN mRNA were measured by ribonuclease protection assay (RPA) with standard curves. Splenius muscle weight was greater than semitendinosus muscle weight in rams compared with wethers at 105, 133 and 161 days (p = 0.05, p = 0.04 and p = 0.02). The difference in IGF-I mRNA levels between the two muscles was higher in rams than in wethers at 133 and 161 days (p < 0.05) and the difference in AR mRNA levels was higher in rams than in wethers at 105, 133 and 161 days (p < 0.05), with higher expression in the SP. No difference was found in the MSTN mRNA level between the two muscles in rams and wethers at any age. These results show that locally produced IGF-I and the regulation of AR expression are important for sexually dimorphic muscle growth patterns.