The combined effects of age and of diet deficient in n-3 fatty acids on delta 6 desaturation of linoleic acid and on lipid fatty acid composition were studied in the liver of the rat at 2, 6, 12, 18 and 24 mon of age. The profiles of delta 6 desaturase activity and fatty acid composition were studied in the deficient rats refed, at these different ages, either with 18:3n-3 (mixture of peanut and rapeseed oils) or with 20:5n-3 + 22:6n-3 (fish oil) diets for 2, 4, 8 or 12 wk. Results showed that the liver delta 6 desaturation activity in the control rats remained high at 2 and 6 mon, decreased by 30% from 6 to 12 mon, and then remained stable from 12 to 24 mon. In the deficient rats, this activity remained high during the entire period studied. Thus, the profile of liver delta 6 desaturase activity after puberty was not related to age only; it also depended on the polyunsaturated fatty acid (PUFA) n-6 and n-3 balance in the diet. In the controls, in parallel with the delta 6 desaturase activity, PUFA metabolism could be divided into three periods: a "young" period, and "old age" period, separated by a period of transition between 6 and 12 mon. Recovery from PUFA n-3 deficiency occurred at all ages but in a different manner depending on whether the rats were "young" or "old". Recovery was faster if long-chain n-3 PUFA rather than alpha-linolenic acid were supplied in the diet.