Sarcomeric muscles including the myocardium respond to increased workload with adaptation by differential gene expression and without cell division. This is characteristic for postmitotic tissues. We showed that a short period of high intensity endurance training induces a shift from fast myosin heavy (HC) and light (LC) chain isoforms towards the slow variety within histochemically typed fibres of human vastus lateralis muscle. In an intermediary state of transition both varieties of isoforms are co-expressed in one fibre. Likewise a shift from the fast myosin alpha-HC towards the slow beta-HC was observed in the atria of severely insufficient human hearts. Such a shift does not occur in the ventricular myocardium where the beta-isoform is expressed mainly. However, both ventricular myosin LC species do occur in the atrium while only the atrial LC-1 isoform appears in the ventricular muscle under increased workload. The expression of the myosin HC and LC isoforms is regulated independently of one another and tends to change towards the slow myosin type with its greater contraction economy.