To determine the synthesis of α-linolenic acid esterified to galactolipids, etiolated leaves from Avena sativa L. were fed with [1-14C]acetate for the first 3 hr of greening, and the redistribution of 14C incorporated into the fatty acid moieties of lipid classes was examined during a 21-hr chase. Phosphatidylcholine (PC) was most heavily labeled, but lost a large portion of its 14C during the chase. Galactolipids, such as monogalactosyl-diglyceride (MGDG) and digalactosyldiglyceride (DGDG), were only slightly labeled at the start, but gradually gained 14C during the chase. When greening Avena leaves were pulse-labeled with [2-3H]glycerol and chased in the same manner, a similar incorporation and redistribution of 3H were observed in the glycerol moieties of lipid classes. A [1-14C]oleic acid-feeding experiment also showed the same redistribution of 14C from PC to galactolipids and little change of the 14C incorporated into phosphatidylglycerol and phosphatidylethanolamine. These results are evidence that galactolipids were synthesized from PC in greening Avena leaves.