Soleus muscle strips from Wistar rats were preincubated with palmitate in vitro before the determination of insulin-mediated glucose metabolism in fatty acid-free medium. Palmitate decreased insulin-stimulated glycogen synthesis to 51% of control in a time- (0-6 h) and concentration-dependent (0-2 mM) manner. Basal and insulin-stimulated glucose transport/phosphorylation also decreased with time, but the decrease occurred after the effect on glycogen synthesis. Preincubation with 1 mM palmitate, oleate, linoleate, or linolenate for 4 h impaired glycogen synthesis stimulated with a submaximal physiological insulin concentration (300 microU/ml) to 50-60% of the control response, and this reduction was associated with impaired insulin-stimulated phosphorylation of protein kinase B (PKB). Preincubation with different fatty acids (all 1 mM for 4 h) had varying effects on insulin-stimulated glucose transport/phosphorylation, which was decreased by oleate and linoleate, whereas palmitate and linolenate had little effect. Across groups, the rates of glucose transport/phosphorylation correlated with the intramuscular long-chain acyl-CoA content. The similar effects of individual fatty acids on glycogen synthesis but different effects on insulin-stimulated glucose transport/phosphorylation provide evidence that lipids may interact with these two pathways via different mechanisms.