Weanling rats were used to examine the role of leucine in in vitro protein turnover in skeletal muscles. In three experiments, rats were subjected to 24 or 72 hours of food deprivation or 5 days of consuming a protein-free diet or injection of streptozotocin. The soleus and extensor digitorum longus muscles were removed and utilized for measures of protein synthesis or degradation by using the isolated, incubated muscle technique of Li et al. These experiments demonstrate that supplementation of the incubation media with 0.5 mM leucine stimulates protein synthesis in these catabolic muscles and that during total starvation the stimulation decreases as the severity of the condition increases. Leucine supplementation failed to affect protein degradation in these skeletal muscles. This study demonstrates that the branched-chain amino acid leucine has the potential to stimulate protein synthesis in skeletal muscles, at least under specific catabolic conditions, but does not affect protein degradation in skeletal muscles under the conditions studied.