The uptake of [2-14C]glycine by leaf discs of Perilla is shown to be a passive process which takes place primarily through the cut edges of the discs. Penetration of the label towards the centre of the discs proceeded only slowly and even after 24 hours on a radioactive solution the amount of isotope in the central region of the disc was no more than 30% of that in an equal area of the outer region. Incorporation of 14C was stimulated by light and increased temperature and partially inhibited by chloramphenicol. The total uptake of label into the discs increased markedly in the course of senescence; this factor may provide useful information concerning the change in permeability of the tissues with age but makes it difficult to draw any firm conclusions concerning the rates of protein synthesis in the course of senescence. The data obtained are discussed and serious doubts cast on the qualitative or quantitative value of many studies involving the use of leaf discs.