Different 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.