We have used blot hybridization techniques and a specific anti-sense RNA probe to determine whether variation in myoglobin gene expression among mammalian striated muscles is attributable to pretranslational regulatory events. We observed that myoglobin mRNA was expressed to approximately 10- and 5-fold greater levels, respectively, in cardiac and soleus (slow-twitch, oxidative, skeletal) muscles of adult rabbits than in tibialis anterior (fast-twitch, glycolytic, skeletal) muscles. Furthermore, when oxidative capacity of tibialis anterior muscles was increased by 21 days of indirect electrical stimulation, a model of exercise conditioning, myoglobin mRNA content increased approximately 15-fold. We conclude that pretranslational mechanisms are important in regulation of myoglobin gene expression in mammalian muscles.