Changes in myosin heavy chain (MHC) mRNAs were studied in rabbit fast-twitch muscles during continuous electrical stimulation at 10 Hz for periods up to 3 weeks, and during the first 12 days of the recovery process that followed cessation of 6 weeks' stimulation. Two cDNA probes were used to detect MHC mRNAs specific to fast- and slow-twitch skeletal muscle in RNase protection assays and Northern- and slot-blot analyses. The isolation and base sequence of one of these probes, corresponding to the MHC gene expressed in soleus (slow-twitch), is described. At an early stage of the response to stimulation, fast MHC mRNA was replaced by slow MHC mRNA. During recovery, this process occurred in reverse but took longer. The time course of recovery was slightly faster in tibialis anterior than in extensor digitorum longus. The changes in mRNAs during both stimulation and recovery reflected changes in the corresponding muscle proteins.