Fruit of Olea europea L. was examined by light and electron microscopy to determine whether commencement of lipid accumulation depended upon the fruit achieving structural maturity. Maturation of fruit develops progressively from the smallest changes towards the largest in cellular structures. Important metabolic and structural changes have been observed: oil body formation, changes in the structural and reserve lipid biosynthesis and in the fatty acid of total lipid content, as well as in G6PDH and LOX activities. The labelling of fruit lipids by previously incubating the leaves with (1-14C)-acetate and (1,5-14C)-citrate or by putting the labelled substrates directly on the fruit surface, shows a 14C assimilate derived from acetate greater than that from citrate; the incorporation of the latter is higher in the methanol-water fractions. At the beginning of fruit development the lipid biosynthesis with both substrates is greater in polar lipids; on the contrary, the incorporation of 14C into neutral lipids increases during fruit maturation. Additionally, a maximum of substrate export from leaves to fruit coincides with an increase in the lipoxygenase and, above all, in the glucose-6-phosphate dehydrogenase activities. The transported 14C from leaves begins its activity before the small oil bodies close to the tonoplast can be observed in the fruit, and well before the beginning of maturation. The results suggest that structural development and some other rate controlling metabolic steps can govern the initiation of lipid accumulation in olive fruit.