Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export Dec.reased considerably at night. Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.