Two new bismacrocyclic Gd3+ chelates containing a specific Ca2+ binding site were synthesized as potential MRI contrast agents for the detection of Ca2+ concentration changes at the millimolar level in the extracellular space. In the ligands, the Ca(2+)sensitive BAPTA-bisamide central part is separated from the DO3A macrocycles either by an ethylene (L-1) or by a propylene (L-2) unit [H(4)BAPTA is 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; H(3)DO(3)A is 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid]. The sensitivity of the Gd3+ complexes towards Ca2+ and Mg2+ was studied by H-1 relaxometric titrations. A maximum relaxivity increase of 15 and 10% was observed upon Ca2+ binding to Gd2L1 and Gd(2)L2, respectively, with a distinct selectivity of Gd2L1 towards Ca2+ compared with Mg2+. For Ca2+ binding, association constants of log K = 1.9 (Gd2L1) and log K = 2.7 (Gd2L2) were determined by relaxometry. Luminescence lifetime measurements and UV-vis spectrophotometry on the corresponding Eu3+ analogues proved that the complexes exist in the form of monohydrated and nonhydrated species; Ca2+ binding in the central part of the ligand induces the formation of the monohydrated state. The increasing hydration number accounts for the relaxivity increase observed on Ca2+ addition. A H-1 nuclear magnetic relaxation dispersion and O-17 NMR study on Gd2L1 in the absence and in the presence of Ca2+ was performed to assess the microscopic parameters influencing relaxivity. On Ca2+ binding, the water exchange is slightly accelerated, which is likely related to the increased steric demand of the central part leading to a destabilization of the Ln-water binding interaction