Photosynthesis rate, internal CO2 concentration, starch, sucrose, and metabolite levels were measured in leaves of sugar beet (Beta vulgaris L.) during a 14-h period of sinusoidal light, which simulated a natural light period. Photosynthesis rate closely followed increasing and Dec.reasing light level. Chloroplast metabolite levels changed in a manner indicating differential activation of enzymes at different light levels. Starch levels Dec.lined during the first and last 2 hours of the photoperiod, but increased when photosynthesis rate was greater than 50% of maximal. Sucrose and sucrose phosphate synthase levels were constant during the photoperiod, which is consistent with a relatively steady rate of sucrose synthesis during the day as observed previously (BR Fondy et al. [1989] Plant Physiol 89: 396-402). When starch was being degraded, glucose 1-phosphate level was high and there was a large amount of glucose 6-phosphate above that in equilibrium with fructose 6-phosphate, while fructose 6-phosphate and triose-phosphate levels were very low. Likewise, the regulatory metabolite, fructose, 2,6-bisphosphate was high, indicating that little carbon could move to sucrose from starch by the triose-phosphate pathway. These data cast doubt upon the feasibility of significant carbon flow through the triose-phosphate pathway during starch degradation and support the need for an additional pathway for mobilizing starch carbon to sucrose.