Linoleic acid plasma kinetics in pregnant baboons and its conversion to long chain polyunsaturates (LCP) in fetal organs is characterized over a 29-day period using stable isotope tracers. Pregnant baboons consumed an LCP-free diet and received [U-13C]linoleic acid (18:2*) in their third trimester of gestation. In maternal plasma, 18:2* dropped to near baseline by 14 days post-dose, while labeled arachidonic acid (20:4*) plateaued at 10 days at about 70% of total labeled fatty acids. After 2;-5 days, total tracer fatty acids decreased in visceral organs, but increased in the fetal brain. Maximal fetal incorporation of 18:2* was 1;-2 days post-dose; thereafter it dropped while 20:4* increased reciprocally. Labeled 20:4 replaced 18:2* in neural tissues by 5 days post-dose. In liver, kidney, and lung, 20:4* became dominant by 12 days, but in heart the crossover was >29 days. Fetal brain 20:4* plateaued by 21 days at 0. 025% of dose, while fetal liver 20:4* was constant from 1 to 29 days at 0.006% of dose. Under these dietary conditions we estimate that the fetus derives about 50% its 20:4 requirement from conversion of dietary 18:2, with the balance from maternal stores, and conclude that 1) fetal organs accumulate 18:2 within a day of a maternal dose and convert much of it to 20:4 within weeks, 2) modest dietary 18:2 levels may support fetal brain requirements for 20:4, and 3) the brain retains n;-6 fatty acids uniquely compared with major visceral organs.