The kinetics of fatty acid (FA) replacement in different membrane compartments in the rat liver were investigated using diets with varying n-6/n-3 FA ratios. Rats at different stages of growth, i.e. after weaning and at 150 g body weight, were either fed a modified AIN 76A diet containing sunflower oil as fat source or the same diet containing sunflower oil and fish oil to achieve n-6/n-3 FA ratios of 12:1 and 6:1 (diets A and B, respectively). In the adult rats, fed diet A for 8 weeks, C18:2n-6 increased significantly at week 2 in the phosphatidylcholine (PC) fraction of the plasma membranes, microsomes and plasma but not in phosphatidylethanolamine (PE), C20:3n-6 increased significantly at week 2 in the plasma membrane and microsomal PC, but did only increased in PE of both compartments by week 8. C20:4n-6 and the n-3 FAs significantly decreased and increased, respectively, at week 2 in PC and PE of both membrane compartments and plasma PC. The experimental diets led to a change in the plasma membrane fluidity but not in the microsomes. The FA changes in the weaned rats followed a similar pattern as in the adult rats although the changes were greater, depending on the phospholipid fraction and specific FA. The decrease in C20:4n-6 was significantly greater in the microsomal PC and PE and plasma PC but not in the plasma membrane PC and PE. The n-3 FAs increased significantly above the adult levels in the plasma membrane PC and PE respectively but not in the microsomal phospholipid fractions. A plateau for maximal n-3 and n-6 FA incorporation was achieved in the adult rats fed diet A in the microsomes after 2 weeks with no further alterations occurring with diet B. In the plasma PC and plasma membranes most of the n-3 FAs achieved a threshold incorporation after 2 weeks on diet A, except for C22:6n-3 in the plasma membranal PE and certain n-6 FAs in the plasma membrane PC and PE. The present data shows that differences exist in the kinetics of FA incorporation and replacement depending on the specific phospholipid fraction, membrane compartment, age and to a certain extent the dietary n-6/n-3 FA ratio.