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We investigated the role of photosynthesis in the photocontrol of extension growth of the hypocotyl of light-grown Cucumb sativw L. Previous work [Gaba and Black (198513) Planr Physiol. 79,10111 demonstrated that the inhibition of cucumber hypocotyl elongation is a fluence rate dependent response in red light. However, the relative contributions of phytochrome and photosynthesis to the photon flux dependent inhibition response were not clear. Here we have shown that photoperception by the foliar cotyledons as well as the hypocotyl itself are responsible for tluence rate dependence in red light. The inhibitor of photosynthesis diuron [3-(3,4-dichlorophenyl)-l, l-dimethylurea] reduced both the magnitude of inhibition and the fluence rate dependency in red light, indicating an involvement of photosynthesis. Furthermore, the growth of non-pigmented seedlings (treated with the herbicide nortlurazon) was less inhibited by red light, with no fluence rate dependency. In particular, inhibition due to cotyledon photoperception was completely lost in non-pigmented (nortlurazon-treated) plants, and much reduced by diuron treatment. Hypocotyl-perceived red light inhibition was only slightly reduced by treatment with nortlurazon and diuron. Photosynthesis was compared directly to photoinhibition of growth: the photon flux response curve of oxygen evolution of green Cucumb cotyledons was distinctly different from that of hypocotyl inhibition. In conclusion, photosynthesis is an essential requirement for the cotyledon-perceived inhibition, but the response itself is not due to photosynthesis.