Treatment of isolated myofibrils with an ATP-containing relaxing solution results in the dissociation of a preformed quantity of myofilaments called 'easily releasable myofilaments'. Van der Westhuyzen, Matsumoto & Etlinger [(1981) J. Biol. Chem. 256, 11791-11797] presented experimental evidence that these myofilaments represent intermediate products in the turnover of myofibrillar proteins. To investigate further this question, we measured the size of the fraction of easily releasable myofilaments in three different species of skeletal muscles from rats subjected to well-defined catabolic conditions, namely starvation or chronic glucocorticoid administration. The results were as follows: (1) The amount of easily releasable myofilaments was transiently increased about 2-3-fold during both experiments, and thus paralleled the known alterations in the rate of overall muscle protein breakdown rather than in those of synthesis. (2) These changes were observed in muscles containing predominantly fast-twitch fibres, but not in slow-twitch soleus muscle, a muscle that is known to be more resistant to catabolic conditions. (3) The starvation-induced increase of the size of the fraction of easily releasable myofilaments could be significantly reduced by treatment of the starving animals with the proteinase inhibitor E-64. These results are compatible with the idea that easily releasable myofilaments are intermediates in the degradative pathway of myofibrillar proteins and that a proteolytic step may be involved in the conversion of myofilaments into easily releasable myofilaments.