Net carbon-dioxide exchange and loss of photosynthate-(14)C from leaves of Vicia faba L. were estimated throughout a day (16/8 h light/dark periods) by infrared gas analysis and Geiger-Müller-tube monitoring, respectively. Net photosynthesis reached 95% of the mean maximum rate within 1 h of the start of the photoperiod, and thereafter remained relatively constant before declining during the last 2-4 h of the photoperiod. Respiration was highest at the start of the dark period. The loss of photosynthate-(14)C from leaves exhibited two phases: a rapid phase which was fastest but shortest following (14)C incorporation in the middle of the photoperiod, and a slower phase of much longer duration. Absolute rates of (14)C loss exhibited diurnal trends reaching 95% of the maximum 6 h after the start of the photoperiod and remaining high until darkness, and then declined markedly. Of the total carbon fixed in one 24 h period, about 50% was lost during the current photoperiod, 14% during the dark period and 5% during the next photoperiod. The estimated amount of carbohydrate remaining within the leaf followed a diurnal pattern of accumulation and depletion. The daily course of translocation was not closely related to that of net photosynthesis or carbohydrate concentration.