Increases in the concentration of malonyl-coa in skeletal musclehave been observed in the kkay mouse, an obese rodent with high plasma insulin and glucose levels (1). To assess whether insulin and glucose directly regulate malonyl-coa in muscle, soleus muscles from young rats were incubated with insulin and glucose at various concentrations, and their content of malonyl-coa was determined. In addition, the effect on malonyl-coa of denervation and electrically-induced muscle contractions was assessed. The concentration of malonyl-coa in the soleus, taken directly from a rat fed ad libitum, was 2.0 ± 0.2 nmol/g. In muscles incubated for 20 min in a medium devoid of added insulin and glucose, the concentration was decreased to 0.8 ± 0.2 nmol/g. When the medium contained 0.5, 7.5, or 30 mm glucose, malonyl-coa concentrations were 1.3 ± 0.1, 1.8 ± 0.1, or 2.4 ± 0.2 nmol/g, respectively, in the absence of insulin and 1.7 ± 0.1, 4.6 ± 0.3 and 5.5 ± 0.6 nmol/g. In its presence (10 mu/ml). Compared with its level in a control muscle, the concentration of malonyl-coa increased 3-fold in the soleus 6–8 h after denervation and remained 2-fold higher for = 48 h. In contrast, muscle contractions induced by sciatic nerve stimulation, in vivo, acutely decreased the concentration of malonyl-coa by 30–35%. The results indicate that insulin and glucose, and probably contractile activity, regulate the concentration of malonyl-coa in muscle. They suggest that malonyl-coa is a component of a fuel-sensing and signaling mechanism that responds to changes in the fuel milieu and possibly the energy expenditure of the muscle cell.