Turnover of liver mitochondrial and microsomal phospholipids was studied in rats kept on isocaloric diets containing 4.5, 6, 9, 18% protein (controls) and 36%. Turnover rate and half-life period of mitochondrial and microsomal phospholipids were determined from degradation of membrane fractions of phospholipids labeled with 14C-palmitic acid. Increase or decrease of protein level in the diet resulted in reduced turnover rate of mitochondrial and microsomal phospholipids and their prolonged half-life period in the membranes. Changed life span of phospholipid molecules during feeding the diets imbalanced with respect to proteins may be considered as adaptation reaction to alimentary exposures on the cellular level, this reaction being aimed at preservation of the optimum structure of subcellular membranes. Earlier findings on reduced half-life period of membrane proteins during low-protein (4.5%) and high-protein (36%) diets and the present data allow a conclusion on the independent nature of the mechanisms of degradation of subcellular membrane lipid and protein components.