We have investigated the usefulness of Secondary Ion Mass Spectrometry (SIMS) for studying the tissue distribution of 15N labelling in yeast cells and soybean leaf tissues. The secondary ions best suited for this are 12C14N- and 12C15N-. Using a mass resolution of 6000, all problems of interference by other ions were avoided. The lateral resolution was of the order of 300 nm, i.e. well suited for subcellular studies. The sensitivity was good enough to allow the detection and the mapping of 15N, even when it was present at its natural value concentration of the isotopic ratio of only 0.37%. Using yeast cells at isotopic equilibrium with their nutrient medium, the nitrogen isotopic ratios in the cells were consistent with those in the medium. In the soybean leaf samples, the mapping of 14N and 15N was well correlated with the anatomical structures of the tissues. The mean isotopic ratios (100 15N/14N, at/at), measured in the leaf tissues by SIMS, were slightly below those in the nutrient medium as well as those measured in the leaf tissue by conventional mass spectrometry. This may be explained by differences in the methods of preparation of the leaf samples for SIMS and for mass spectrometry, and by the fact that the plants were probably still not perfectly at isotopic equilibrium with their external medium at the time the experiments were performed.