The degradation of neutral lipid and the development of lipase activity in cucumber cotyledons is stimulated by white light. Malate synthase and isocitrate lyase activities show no stimulation. Lipase activity and neutral lipid breakdown are also enhanced by red light, far-red light proving ineffective. Far-red light reverses the effect of red light indicating the involvement of phytochrome in the control of lipase activity. Although light stimulates neutral lipid degradation it appears that much of the additional lipid lost is used in the synthesis of polar lipid constituents. Furthermore, the influence of light on lipid degradation appears to be species dependent.