We report the influence of media lipids, growth in lipid-poor medium, and cell differentiation on delta 9, delta 6, and delta 5 desaturase activity in the human CaCo-2 enterocyte cell line. We also describe the level of incorporation of palmitic (16:0), linoleic (18:2n-6), and eicosapentaenoic (EPA) acids (20:5n-3) and their higher homologues into cytosolic and membrane lipids during long-term (10 days) medium supplementation in fully differentiated 16- to 18-day-old cultures. CaCo-2 monolayers reached confluency by day 6 with subsequent development of microvilli and maximal expression of microvillus membrane sucrose, alkaline phosphatase, and gamma-glutamyltransaminase occurring between days 16 and 23 after plating. There was evidence of the presence and modulation of delta 9, delta 6, and delta 5 desaturase activity (delta 9 > delta 6 > delta 5). delta 6 Desaturase activity decreased approximately 2-fold between days 6 and 24 of culture and when the fetal bovine serum concentration was increased from 0.5% to 25%; in contrast, when cells were starved for 72 h, activity increased 5.4-fold. When the media was supplemented with either linoleic acid and/or EPA, both delta 6 and delta 5 desaturase activities were inhibited, the greatest reduction of delta 5 desaturase activity occurring with EPA. Incorporation of media fatty acids plus their desaturase and elongase products was highly dependent on medium composition with the homologues of delta 9 > delta 6 > delta 5. Supplementation of cellular media with 100 microM EPA for 10 days decreased membrane phosphatidylethanolamine arachidonic acid level from 13.2 to 8.9%. From these results we conclude that enterocyte membrane fatty acid composition and desaturase enzyme activity are regulated by both dietary fat intake and cell maturation. The clinical relevance of these observations on lipid dietary modification for the management of chronic inflammatory bowel disease is still uncertain but these observations suggest that the beneficial effects of EPA supplements on human ulcerative colitis may be due to a reduction in enterocyte arachidonic acid content by down-regulation of delta 6 and delta 5 desaturase activity.