The University of Chicago high resolution scanning ion microprobe (UC-SIM) was used to image, by Secondary Ion Mass Spectrometry (SIMS), the distribution of Ca2+ and Mg2+ in the chromosomes of Indian muntjac (IM) deer mitotic fibroblasts. This is part of a systematic study of the cation composition of mammalian cells and chromosomes throughout the cell cycle, after having shown that Ca2+ and Mg2+ appear to, be important for chromosome condensation and structure at metaphase. We focus here on a detailed description of the metaphase-anaphase transition at narrow time intervals beyond the G2/M border, made possible by controlled cell synchronization procedures. High-density distributions of chromosome spreads showed progressive stages of mitosis, identified by their morphology, within the same UC-SIM field of view. Subtle differences in cation contents between successive mitotic stages could thus be quantified in identical experimental conditions. Preliminary results indicate maximal chromosomal concentrations of Ca2+ and Mg2+ at metaphase, and a progressive decrease of the same with advancing stages of anaphase. Ca2+ and Mg2+ distributions were also imaged in the polytene chromosomes of Drosophila melanogaster, whose DNA distribution had been previously studied by BrdU labeling. These cations may play a common role in mitosis from lower eukaryotes to mammals. (c) 2006 Elsevier B.V. All rights reserved.