Pot experiments were conducted with Hordeum distichon L. (mutant of the cultivar Plena) and Hordeum vulgare L. ssp. hexastichon (cv. Certina). At two stages of development (5 and 20 d after the end of flowering), the assimilate demand of the sink organs (ears) was increased by daikening them for 5 d. The influence of this treatment on the apparent CO2 assimilation (P^), the *'*C02 incorporation of source leaves (flag leaves) and the suhsequent distrihution in the plants were investigated using a combination of gas exchange measurements by IRGA and exposure of the plants to ^'^002. Darkening of the ear 5 d after the end of flowering produced no change in incorporation of the source (flag leáf), or subsequent distrihution to the individual organ fractions in either of the cultivars. Darkening of the ear of cv. Certina 20 d after the end of flowering caused an increased orientation of the translocation to the ear, while of the flag leaf was not significantly influenced. In the Plena cultivar, the loss of ear photosynthesis caused by the darkening could not be compensated by the increased translocation, which was already very high in the control plants; of the flag leaf was enhanced. In the course of grain filling (5 d -> 20 d after the end of flowering), the absolute P^, and protein and chlorophyll contents of the flag leaves strongly decreased in both cultivars. Darkening pf the ear delayed this process in cv. Plena and partly also in cv. Certina. In the čase of an increased assimilate demand of the sink (ear), the CO2 assimilation of the source leaves was reduced only if the saccharide reserves of the plant were depleted. Differences in source-sink interactions observed between cultivars and developmental stages could be explained by this result.