The multicatalytic proteinase purified from rat skeletal muscle was shown by electron microscopy to be a uniform cylinder-shaped protein particle. As the enzyme activities are enhanced by free fatty acids in vitro, we tested, by in vivo perfusion, whether high concentrations of free fatty acids increased the multicatalytic proteinase activities in skeletal muscle tissue. Perfusion of rat hindquarters with plasma containing 1.5 mM oleic acid bound to albumin led to a 30-40% increase of the proteinase activities in gastrocnemius muscle. This increase was statistically not significant when compared to control rats perfused with plasma containing fatty acid free albumin. These results are discussed under the following aspects: 1. the in vivo concentration of non-esterified fatty acids within the muscle cell is not known; 2. the uptake of free fatty acids by the muscle cell is too low, or the metabolism of free fatty acids taken up by the cell is too fast to allow activation of the multicatalytic proteinase; 3. the free fatty acids are bound to other proteins (e.g. fatty acid binding protein), which abolish their ability to activate the proteinase.