The linoleic acid metabolism was studied in serially cultured human skin fibroblasts during 30 passages. Confluent cells were incubated with [U-14C]linoleic acid in serum free medium for 4 h after 3, 6, 12, 18, 24 and 30 passages. The labeled linoleic acid was rapidly incorporated in cell lipids, preferentially into phospholipids (80%), and the distribution within the lipid classes remained constant during the cell cycles. The rate of conversion of [14C]linoleic acid into long chain fatty acids varied with the number of passages. The formation of dihomo-gamma-linolenic acid and of arachidonic acid decreased significantly from 12% to 5% (as % of incorporated linoleic acid) between 3 and 30 serial passages whereas the linoleic acid content increased in the cells from 68% to 83%. The results indicate that the desaturase activities decreased significantly during cellular aging of human skin fibroblasts.