Substances which can perturb the transmembrane cation balance in a predictable manner have wide-ranging uses in the study of cellular processes. Transmembrane Ca transport was examined on the molecular level through the design and synthesis of a series of ionophoric peptides as models for protein-mediated Ca transport. General mechanisms for carrier-mediated membrane transport are discussed. Cation transport profiles are presented for the transport by synthetic peptides of structure cyclo(Glu(OR)-Sar-Gly-(N-R1)-Gly)2, where R = benzyl ester or H; R1 = n-decyl or cyclohexyl. Transport of physiologically abundant cations across liquid membranes in Pressman cells mediated by cyclo(Glu-Sar-Gly-(N-decyl)Gly)2 was essentially Ca specific, as long as Ca ions were present in the system. Multilamellar and unilamellar phosphatidylcholine vesicles were each emptied of internal 45Ca2+ ions upon addition of cyclo(Glu(OBz)-Sar-Gly-(N-cyclohexyl)Gly)2 to the vesicle suspension. The results are compared with the naturally occurring calcium ionophore A23187 [calcimycin].