SummaryDifferent electrically neutral lipophilic di-and triamides were prepared and their ion selectivity in membranes studied. In membranes the ionophore N, N'-diheptyl-N, N'-dimethyl-succinamide prefers Ca2+ over Mg2+ by a factor of 20 but rejects Na+ and K+ in respect to Mg2+ by a factor of 100 and 10, respectively. This selectivity suffices to perform useful intracellular Mg2+ activity studies.Introduction. -The monocarboxylic polyether antibiotic-60 16 has been reported to be a Mg2+ selective ionophore which displays preferential extraction from an aqueous into an organic phase of Mg2+ over Ca2+ and Ba2+ [ 11. We were, however, unable to detect potentiometrically such selectivities when the antibiotic was incorporated in solvent polymeric membranes. In these membranes Ca2+ was clearly favoured over Mg2+. Similar results were obtained for the antibiotic A23187 which was claimed to be Ca2+ selective in extraction [2] as well as in transport systems [3]. The potentiometrically determined selectivity sequence is, however, Ba2+ > Ca2+ > Mg2+ [4j1).Despite major efforts it has so far been impossible to prepare a single membrane system showing a high selectivity for Mg2+ over Ca2+. According to model calculations such a preference for Mg2+ might be achieved by an octahedral coordination of the cation with the 0-atoms present in the high-dipole-moment ligand groups [8]. A number of 3-oxapentane diamides were prepared and tested in membranes [9] to see whether the suspected 1 : 2 catiodligand complexes were formed by a hexadentate coordination. Although N, N, N , N'-tetracyclohexyl-3-oxapentane diamide does indeed form 1 : 2 Mg2+/ligand complexes [9] [lo], in membranes it induces a selectivity of about lo4 for Ca2+ over Mg2+. This can be explained by the observed formation of highly lipophilic 1 : 3 Ca2+/ligand complexes [9].