The development of photosynthetic activity in cotyledons from non-dormant apple embryos and embryos in a state of dormancy was studied during the culture of intact embryos, of single cotyledon with axis, and of isolated cotyledons. The embryonal axis was found to stimulate the development of photosynthetic activity in the cotyledons. In non-dormant embryos the influence of the axis on the assimilation of CO2 by cotyledons became apparent at an early stage of culture; in the dormant embryos it was noted first after 10 to 12 days. No influence of the embryonal axis was observed on chlorophyll biosynthesis in cotyledons. The photosynthetic rate was always higher in cotyledons from non-dormant embryos than in those from dormant ones. However, the chlorophyll content was similar in cotyledons from both groups of embryos. It is concluded that the interaction of the embryonal axis and cotyledons in the development of photosynthetic activity in seedlings depends on the physiological state of the embryo.