Feeding of 15N-nitrate, 15N(amide)-L-glutamine, or 15N-L-glutamic acid to detached shoots of pea through the transpiration stream results in the soluble and insoluble nitrogen of stem, leaves, and fruits becoming extensively enriched with isotopic nitrogen. The time course of labelling suggests that non-reproductive parts are the principal centres of uptake and assimilation and that from them translocation takes place to the developing seeds. Distribution patterns for 15N in free and protein-bound amino acids of leaf and seed indicate that each labelled source donates nitrogen to a wide range of amino compounds, with no evidence of consistent differences in the manner in which each is assimilated. Alanine, glutamic acid, homoserine, and γ-aminobutyric acid, are the main recipients of 15N in the soluble fraction of the leaves, whilst in the insoluble fraction nitrogen of the amino acids serine, glycine, alanine, threonine, glutamic acid + glutamine, and aspartic acid + asparagine achieves high specific labelling. Amino acids of the seeds are labelled more uniformly with 15N. A complementary 14C-labelling experiment on the translocation of photosynthetically fixed carbon from leaf to seed is described and the labelling patterns obtained for amino acids in leaf, seed, and phloem exudate are discussed in relation to those for 15N.