Summary The accumulation of two sesquiterpene lactones studied ( I = niveusin C, II = 15-hydroxy-3-dehydrodesoxyfruticin; Fig. 1) in leaves of sunflower seedlings ( Helianthus annuus L.) is light dependent. In leaves grown in high intensity light (HIL; 100 W · m −2 ), the concentration of sesquiterpene lactone (SQL) I increased 6-fold and of SQL II 19-fold compared to leaves in low intensity light (LIL; 5 W · m −2 ). Fluence rates of more than 100 W · m −2 could not further enhance SQL accumulation. Light-induced generation of compound II was observed after a lag phase of 40 to 60 min, whereas the concentration of compound I remained unchanged within the first 12 h of irradiation. The accumulation rate of SQL II in primary leaves was 3 to 6 μg · h −1 · g fresh weight −1 (fluence rate 100 W · m −2 ; 20 °C; 80 % rel. humidity). Irradiation of a single leaf could not induce the synthesis of SQL in other leaves or parts of the plant. No transport of SQL occurred within the plant or even within the leaf area itself. Irradiation with pulses of red light led to a significant accumulation of compound I and II ; the red light effect could partly be quenched by a subsequent far red pulse. Thus the involvement of phytochrome in SQL biosynthesis is suggested. In addition it was shown, that light induced-formation of SQL occurred also in leaves which were bleached by the herbicide SAN 9789. Therefore, photosynthesis is not primarily involved in the biosynthesis of SQL.