It is well established that legume species differ in intensity and duratio of nitrogen fixation during pod-filling. This was studied on new lines of white and blue lupins by comparing a solely nitrogen fixing and a treatment supplied with15N-labelled fertizlier at flowering in pot experiments. Nitrogen application at flowering markedly incresed seed and total dry matter and seed and total N yield in white lumpins but remained without effect in blue lupins. simply replacing nitrogen fixation. Nitrogen fixation virtually ceased with the onset of pod-filling in white lupins, causing a massive N-remobilization from vegetative plant parts. In contrast, the blue lupins fixed about 70% of total nitrogen fixation after flowering and displayed growth and net-N-assimilation of the vegetative plant parts. In both species the stems and pods especially supported seed N-filling, indicated by a low% N and a high C/N ratio at ripeness. It is concluded that pod-filling proved to be a critical stage for the seed yield in grain legumes. Exploiting differences in nitrogen fixation in this growth stage,e.g., by breeding, needs more precise knowledge about the regulation mechanisms and source-sink relations.