Introduction. The ability of non-invasively observing changes in the Ca2+ concentrations is important in the understanding of a great variety of neuronal processes. Several compounds were designed (Fig.1) to take advantage of the different binding abilities of carboxylates and phosphonates to gadolinium. Furthermore the different affinities of the two functional groups to Ca2+ permit to obtain free coordination sites at gadolinium. The generation of these coordination sites, which are mandatory for water relaxivity, depends on the structure of the complexes and the Ca2+ concentration. Gadolinium complexes 1 and 2 were synthesized and relaxivity studies were performed. Methods. For measurements of the relaxivities r1 and r2, concentrations of 0.1, 0.4, 0.7, and 1 mM of the CA and different concentrations of Ca2+ (0, 1.0, 10.0 and 100.0 mM) were used at pH 7.2 in a KMOPS buffer. Relaxivity r1,2 was calculated from the slope of R1,2(c) versus the CA concentration by an error-weighted linear regression. Results. Changes of relaxivity was observed for the different Ca2+ concentrations (Fig. 2). An increase of the Ca2+ concentration between 10 to 100 mM enlarged relaxivity up to 20%. According to the results shown it is expected that by the incorporation of more structural constraints into the acyclic part (X and Y) changes in relaxivity can be moved towards the range of physiological conditions (i.e. 1mM in the extracellular space).