In greenhouse pot trials, L. perenne cv. Barlet plants were labelled with 13C at regular intervals from main spike emergence onwards in order to identify and measure the activity of source and sink organs during seed formation. The source activity of the various tiller groups within the plant roughly reflected the relative contributions of these groups to total plant dry mass. After anthesis there was little net exchange of 13C-label between the older and younger tiller groups. From main spike emergence onwards the source activity of the leaves of the reproductive tiller declined sharply, from 95% of total tiller photosynthesis to 16% at final cutting. The spike became the main assimilating organ on the flowering tiller as the leaves aged. During anthesis the stem was a stronger sink than the seeds. At final cutting 70% of the label was located in the stem, when fixed during anthesis. Water-soluble carbohydrates accumulated in the stem, forming up to 25% of dry matter. After anthesis the sink strength of the developing seeds increased and that of the stem decreased and the stem remained a net sink organ up to about mid-seed filling. Pre-anthesis assimilates contributed 14% to final seed and spikelet carbon, when correcting for the palea and lemma that are present before anthesis. These results show that the stem is a temporary storage organ that can support seed filling. However, only a small amount of the stem reserves was used by the seeds. In contrast to carbon, nitrogen was largely redistributed from the stem and leaves to the seeds. At final harvest 59% of the nitrogen in the flowering tiller was located in the seeds.