Background and objectives: L-carnitine (LC) is a non-protein amino acid includes both diet and endogenous synthesis. There are no studies on the effect of LC on glucose uptake and net glucose output in muscle and hepatic tissues. So, the aim of the present work is to study the metabolic aspects of LC and to study some mechanisms that may be of help in treatment of insulin resistance. Methods: Twenty four male rats were divided into two main groups, twelve rats per each group. In vivo group was divided into control one and L-Carnitine - treated one, It was given in the dose of 3 gm carnitine/kg once every day orally for 4 weeks where fasting serum glucose and plasma free fatty acids (FFA) were measured. In vitro group included experiments on epididymal fat pad (EFP), hemidiaphragm and liver. Results: LC caused a highly significant decrease in fasting plasma FFA level (4.9 ± 1.5 mg/ dl). However, it caused a significant increase in insulin-stimulated glucose uptake by EFP in the presence of supraminimal and supramaximal insulin concentrations (2.5 ± 0.8 and 3.4± 0.4) (mg/ gm wet tissue /hour) and also by hemidiaphragm (4.9 ± 0.3 and 6 ± 0.97) (mg/ gm wet tissue /hour). Also, it increased net glucose output by liver slices in the presence of both insulin concentrations (- 4.9 ± 1.1 and – 4 ± 1.42) (mg/ gm wet tissue /hour). It decreased FFA release from EFP in the presence of both insulin concentrations (0.5 ± 0.21 and 0.4 ± 0.2 ) (mg/ gm wet tissue /hour), hemidiaphragm (0.5± 0.1 and 0.5 ±0.16) (mg/ gm wet tissue /hour) and liver slices (0.58 ± 0.19 and 0.63± 0.27) (mg/ gm wet tissue /hour). Conclusion: LC increased plasma FFA level. It potentiated the effect of insulin on glucose uptake in EFP and hemidiaphragm. It decreased hepatic insulin sensitivity and responsivity and enhanced the suppressive effect of insulin on FFA release from different peripheral tissues.