Abstract Plasma membrane fractions were prepared from young (10-day-old), mature (17-day-old) and senescent (10 + 7-day-old) oat leaves ( Avena sativa L.) through aqueous two-phase partitioning and analyzed for their free sterol composition. Continued leaf growth resulted in a strong increase in overall sterol content from 40 nmol · mg −1 lipid to about 120 nmol · mg −1 lipid (1.5% and 5% by wt. of total lipids respectively). This increase could be completely ascribed to increased cholesterol and stigmasterol concentrations. Kinetic parameters of vanadate-sensitive ATPase and NADH-FeCN reductase activities did not change significantly with leaf maturation. The sterol concentration increased to a lesser extent (4.2% and 4.4% by weight) in the plasma membrane of 10-day-old leaf segments that were cut and kept in light or dark for 7 more days. However, the increase in this case was completely due to a raise in Δ 7-avenasterol and 28-isofucosterol concentrations. The K m and V max values for the ATPase activity decreased significantly in these membranes. The characteristics of the FeCN 2 reductase and of glucan synthetase II activity were apparently not affected. The effect of senescence on sterol biosynthesis and on plasma membrane enzyme activities is discussed.