Two novel Gd(III) complexes with functionalised polyaminocarboxylate macrocycles, 1,4,7-tris(carboxymethyl)-9,24-dioxo-14,19-dioxa-1,4,7,10,23- pentaazacyclododecane (L(1)) and 1,4,7-tris(carboxymethyl)-9,25-dioxo-14,17,20-trioxa-1,4,7,10,23- pentaazacyclotridecane (L(2)), were prepared in good yield. Their potential use as magnetic resonance imaging (MRI) contrast agents (CAs) was evaluated by investigating their relaxation behaviour as a function of pH, temperature and magnetic field strength. The 1/T(1) proton relaxivities at 20 MHz and 25 degrees C of GdL(1) (5.87 mM(-1) s(-1)) and GdL(2) (6.14 mM(-1) s(-1)) were found to be significantly higher than the clinically used Gd 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Gd(DOTA)(-)) and Gd diethylenetriaminepentaethanoic acid (Gd(DTPA)(2-)). The complexes possess one water molecule in the inner coordination sphere whose mean residence lifetime was estimated to be 1.1 and 1.5 micros at 25 degrees C by variable temperature (VT) (17)O NMR spectroscopy.