Introduction. As calcium plays an important role in regulating a great variety of neuronal processes, many efforts are already made to generate gadolinium complexes that can act as a calcium-sensors in MRI. We developed a series of the DO3A-based macrocyclic and bismacrocyclic gadolinium chelates, bearing phosphonate groups as an additional coordination sites. These complexes are hypothesized to change the MRI contrast dynamically with Ca2+ concentration. Different lengths of the phosphonate side chains are exploited for fine-tuning the sensitivity of the agent to calcium ion concentration. Methods. The macrocyclic ligands DO3A-alkyl aminobis(methylenphosphonates) (alkyl = propyl 1; amyl 2; hexyl 3), and the bismacrocyclic bisDO3A-ethyleneaminobis (alkyl)phosphonates (alkyl = methyl 4; ethyl 5) were prepared in multistep reactions from cyclen. Gadolinium complexes were obtained by treating the ligands with GdCl3 at pH 7.0 -7.5 and 70°C for 24h. Longitudinal and transverse relaxivities r1 and r2 were measured on a vertical 7 T/60 cm imaging system, at pH 7.4 and different concentrations of Ca2+. Results. The sensitivity of the complexes 1-3 for the changes in Ca2+ concentrations increased with the chain length between the DO3A unit and the phosphonate functions (Fig. 1). Maximal observed change in relaxivity in presence of the physiological extracellular concentration of calcium (~1 mM) was up to 35% for compound 3, and was selective to Ca2+ compared to other physiologically important metals such as magnesium or zinc. The significant changes in the relaxivity of the bismacrocyclic compounds 4 and 5 were found in the presence of a higher concentration of Ca2+ ions. This work is supported by th