The incorporation of radioactivity from orally administered γ-linolenic acid-1-14C and linoleic acid-3H into the liver, plasma, and brain lipids of suckling rats was studied. Significantly more radioactivity from the former compound was incorporated into the liver and brain lipids 22 hr after dosing. The distribution of the radioactivity in the fatty acids of the liver and brain lipids was different for each isotope. Most of the3H was still associated with linoleic acid, whereas most of the14C was in the 20:3 and 20:4ω6 fractions. These results suggest that the desaturation of linoleic to γ-linolenic acid in vivo is a rate-limiting step in the conversion of linoleic to arachidonic acid.