Inactivation of v-src in transformed chicken myoblasts induced changes in expression of the myogenic regulatory factors (MRF's), MyoD, myogenin and Myf5. Both MyoD and Myf5 were expressed in the transformed cells. The inactivation of v-src by a temperature up-shift resulted in the loss of Myf5 expression, a transient increase in MyoD, followed by a rise in myogenin mRNA levels. The increase in transcripts for the later muscle proteins, myosin heavy and light chains, alpha-actin and troponin T followed the sequence of changes in MRF expression suggesting that their induction was the result of altered MRF expression. Thus, v-src appears to operate through effects on MRF's and not directly on the expression of the late muscle-specific proteins. When v-src was reactivated in mature myotubes by a temperature down-shift, the pattern of expression was quite different, the late muscle-specific transcripts decreased before the loss of myogenin transcripts and before the rise of Myf5 transcripts.