The goal of this review is to summarize our knowledge of the plasticity of striated muscles in terms of contractile proteins. During development or when the working conditions are changed, the intrinsic physiological properties of both cardiac and skeletal muscles are modified. These modifications generally adapt the muscle to the new environmental requirements. One of the best examples is compensatory overload obtained in fast skeletal muscle by synergistic tenotomy and in a fast ventricle, such as in rats, by aortic banding. In both cases, after a few weeks the initial speed of shortening for the unloaded muscle drops, whereas the maximum tension developed remains unchanged. Heat measurements show that efficiency (i.e., g work/mol ATP) is improved at the fiber level. The fast skeletal muscle becomes slow, fatigue resistant, and then more adapted to endurance. For the ventricle as a whole to become slow is beneficial only if one contraction is considered; however, it is detrimental in terms of cardiac output and leads finally to failure. This adaptational process is partly explained by quantitative and qualitative changes in contractile proteins. Protein synthesis is rapidly enhanced and muscles hypertrophy, which in turn multiplies the contractile units and for the cardiac cylinder normalizes the wall stress. In the meantime the structure and, for myosin, the biological activity of several contractile proteins are modified. These modifications are very unlikely to be posttranscriptional and are in fact explained by several isoform shifts. In both tissues, for example, the expression of the gene coding for a fast myosin (MHCf in skeletal muscle, alpha-MHC in ventricles) is repressed and that of the gene coding for a slow myosin (beta-MHC in both tissues) is stimulated. This is accompanied by a coordinated increase in synthesis of other contractile proteins and, in skeletal muscle only, by isoform shifts of myosin light chains and of the TM-TN regulatory system. Other changes are less well understood. During development it has recently been discovered that three different MHCs (MHCemb, MHCneo, and MHCf) appear sequentially in fast skeletal muscle, which explains, for example, several contradictions of immunological cross-reactions. Currently, however, the functional significance of this finding is unknown, and the well-known decrease of shortening velocity observed in cardiac and skeletal muscles during fetal life is unexplained in terms of contractile proteins.(ABSTRACT TRUNCATED AT 400 WORDS)