1. 1. Rat-liver mitochondria take up Mg2+ in exchange for H+. 2. 2. Inorganic orthophosphate is not essential for Mg2+ uptake. 3. 3. Inorganic orthophosphate uptake can take place in the absence of Mg2+. 4. 4. Rat-liver mitochondria take up Ca2+, in part at least, by the same route as Mg2+. H+ is exchanged for Ca2+. 5. 5. The divalent cations appear to share a common transport mechanism with K+. 6. 6. It is concluded that mitochondrial cation-transport mechanism is a system which extrudes H+. Any one of several cations may be taken up in exchange. 7. 7. An explanation is advanced for the opposing effects of 2,4-dinitrophenol and Ca2+ on mitochondrial swelling. 8. 8. The sensitivity and specificity of atomic absorption spectrophometry facilitated analysis of Ca2+ and Mg2+.