The hepatic lipid content and composition were assessed in healthy mice throughout adulthood. It was found that aging and obesity contributed to increase the amount of lipids in the liver while decreasing the poly-unsaturation degree. The combination of in vivo 1H MRS assessments with in vitromeasurements on tissues extracts illustrated the important contribution of membrane lipids to the total poly-unsaturation degree of the fatty acyl chains. Changes of the unsaturation profile of cytosolic lipids can be monitored by in vivo 1H MRS, which is of interest for the study of genetic and diet-induced mice models of metabolic diseases.