The synthesis of two gadolinium(III) complexes that exhibit an increase in proton relaxivity in the presence of added Ca2+ or Zn2+ ions is reported. The complexes increase their hydration state from zero to one following metal-ion binding, confirmed by spectral measurements on the corresponding EuIII complexes. At a field of 1.4?T and 310?K, modulation of relaxivity of the order of 30–40?% was observed in mouse serum in each case. The dissociation constants for Ca2+ and Zn2+ binding were sensitive to the presence of added bicarbonate, and were 450?μM (Ca2+) and 200?μM (Zn2+) in serum. Such systems may, therefore, be considered for use as magnetic resonance imaging (MRI) contrast agents to track the restoration of changes in metal-ion concentration in the cerebrospinal fluid of the brain, following neural stimulation.