Using a reverse transcription/polymerase chain reaction method, the temporal pattern of expression of the myogenic regulatory genes (myf5, myogenin, MRF4, myo D) was quantitated in developing mouse muscle (whole embryo: 6.5 to 12.5 days postcoitum (dpc); front limb buds: 9.5 to 12.5 dpc; hind limb buds: 11.5 to 14.5 dpc) and related to expression of TGF-beta 1, b-FGF, IGF-I, and IGF-II. Myf5 was the first myogenic regulatory factor to appear in both the whole embryo and front limb bud, with expression evident 7.5 and 9.5 dpc, respectively. A transient peak of MRF4 expression occurred 10.5 dpc in both the whole embryo and the front limb bud. Myogenin and myo D expression in the whole embryo was detected 8.5 and 9.5 dpc, respectively. In the front limb bud myogenin and myo D expression was not detected until 10.5 dpc. In the hind limb bud myf5, myogenin, and MRF4 expression was detected 11.5 dpc. Myo D expression was not detected until 12.5 dpc. With respect to growth factor expression, in the front limb bud TGF-beta 1, IGF-I, and IGF-II were evident 9.5 dpc, while bFGF was not detected until 10.5 dpc. In the hind limb bud TGF-beta 1, bFGF, IGF-I, and IGF-II expression was detected 11.5 dpc. These results show that in both the whole embryo and limb buds, all four myogenic regulatory factors are involved in the initiation of the myogenic program. We also show myf5 expression in the 9.5-dpc front limb bud, suggesting its expression in the somite-derived migrating muscle precursor cells. Correlations between growth factor-mediated myoblast proliferation and myogenic differentiation are discussed.