We report here on the ability of secondary ion mass spectrometry (SIMS) to provide rapid imaging of the intracellular distribution of 14C-labeled molecules. The validity of this method, using mass discrimination of carbon 14 atoms, was assessed by imaging the distribution of two molecules of well-known metabolism, [14C]-thymidine and [14C]-uridine, incorporated by human fibroblasts in culture. As expected, 14C ion images showed the presence of [14C]-thymidine in the nucleus of dividing cells, whereas [14C]-uridine was present in the cytoplasm as well as the nucleus of all cells, with a large concentration in the nucleoli. The time required to obtain the distribution images with the SMI 300 microscope was less than 6 min, whereas microautoradiography, the classical method for mapping the tissue distribution of 14C-labeled molecules, usually requires exposure times of several months. Secondary ion mass spectrometry using in situ mass discrimination is proposed here as a very sensitive method which permits rapid imaging of the subcellular distribution of molecules labeled with carbon 14.