Similar to all other eukaryotic cells and tissues muscle tissue contains the proteolytic system of 20S/26S proteasomes with the 20S proteasome existing predominantly in a latent state. Unlike with the mammalian enzyme in vitro transition from the latent to the activated state of the 20S proteasomes isolated from muscle of several fish species and from lobster can be achieved by heat shock. It is very likely that the activated state of the 20S proteasome corresponds to the physiologically active form of the enzyme since only that one is able to attack sarcoplasmic and myofibrillar proteins to any significant extent. As perfusion of rat hindquarters with presumptive low molecular mass activators like free fatty acids does not result in an activation of the muscle proteasome other--possibly protein activators--may serve this purpose in vivo. The 26S proteasome complex may be regarded as such a proteasome/activator complex. The 26S proteasome complex has the ability to degrade protein (-ubiquitin-conjugates) by an ATP-consuming reaction. Since increased amounts of ubiquitinated proteins as well as an enhanced activity of the ATP (-ubiquitin)-dependent proteolytic system have been measured in rat muscle tissue during various catabolic conditions, it is not unlikely that this pathway is responsible for catalysis of muscle protein breakdown.