Alterations of the fatty acid content and pattern of plasma unesterified fatty acids, phospholipids, triglycerides and diglycerides were examined in second generation rats depleted in long-chain poly-  unsaturated ω3 fatty acids before and 60-120 min after the intravenous injection of either a medium-chain triglyceride: fish oil emulsion or a control medium-chain triglyceride: olive oil emulsion. The ω3-depleted rats displayed higher concentration of all lipid classes. The time-related changes of triglycerides and diglycerides after injection of a lipid emulsion suggested a dual regulatory process, i.e. an increase in concentration resulting from the administration of exogenous lipids and an inhibition of the lipid production from endogenous stores. In addition to the expected decrease in long-chain polyunsaturated ω3 fatty acids, with a low C22:6ω3/C22:5ω3 ratio, in both the phospho-lipids and triglycerides of ω3-depleted rats, these animals displayed a low weight percentage of C18:2ω6 in all lipid classes, an apparently increased activity of Δ9-desaturase as judged from the C16:1ω7/C16:0 and C18:1ω9/C18:0 ratios, and a less efficient conversion of C18:1ω9 to the precursors of nervonic acid. Several of these variables demonstrated age-related changes in the ω3-depleted rats. This information sets the scene for individual comparison between the weight percentage of fatty acids in various organs (such as liver) and circulating lipids in this animal model of insufficient dietary supply of ω3 fatty acids.