One of the perplexing physiological dilemmas encountered during cotton (Gossypium hirsutum L.) production is the decline of leaf and canopy photosynthesis at cutout, a period of maximum boll loading and little vegetative growth. This large reproductive sink development would also seem to necessitate the production of copious amounts of photosynthate, but the opposite actually occurs. Reasons for this decline are unclear; simple canopy aging or to some environmental agent could be the cause. The objectives of this study were 1) to attempt to separate environmental from canopy age effects and 2) to identify some of the photosynthetic components that may be rate limiting during this period.