Summer pumpkin (Cucurbita pepo L.) is an herbaceous plant with big fruits and oily seeds. Photosynthetic oxygen release was previously proposed to promote storage activity of seeds under internal hypoxia, in particular in oilseeds. Photosynthetic activity or any other function of chlorophylls from pumpkin seeds has never been demonstrated. The aim of the present research was to test whether illumination of pumpkin fruits has any influence on the yield and to measure photosynthetic activity of pumpkin seeds. The fruits grown in the dark were not significantly different from controls exposed to light in regard to fruit weight, size, number and weight of seeds or the content of lipids and pigments. Delayed fluorescence measurement showed no photosynthetic activity in pumpkin seeds in any developmental stage. Fruit wall, on the other hand, had significant photosynthetic activity. It is concluded that chlorophyll in pumpkin seeds does not mediate photosynthesis, which would in turn result in increased pumpkin yield.