Metal complexes of polyaminocarboxylic acids are of interest as contrast media for Magnetic Resonance Imaging (MRI). Such metal complexes are stable and will therefore minimize the toxic effects of the free metal ions. Attention has been focused especially on the paramagnetic gadolinium(III)-, iron(III)- and manganese(II)-ions. The gadolinium complexes appear to be the most potent but the biological effects of the free metal are of some concern. In contrast the biological effects of iron are well known and iron(III) complexes are therefore of particularly interest as MRI contrast agents. In this study the iron(III) complexes of diethylenetrinitrilopentaacetic acid (DTPA), trans-1,2-cyclohexylenedinitrilotetraacetic acid (CDTA), triethylenetetranitrilohexaacetic acid (TTHA) and ethyulenediiminobis(2-hydroxyphenyl)acetic acid (EHPG)were investigated. The technique used was the electroanalytical method differential pulse polarography (DPP). This technique is particularly well suited to such studies because of its ability to discriminate between the free metal ion, the metal complex and the free ligand. The complexes of DTPA, CDTA and TTHA were found to decompose slowly at pH7,2. With TTHA a mixture of 1:1 and 2:1 complexes was obtained and the transformation between the two complexes was slow. EHPG was found to be the most suitable ligand. The complex formation is kinetically slow, and a special procedure for the preparation of the complex is required. The complex is then stable for one week at pH 7,2. The polarographic determination can be done in the presence of 10% urin or serum without interference. The detection limit is in the low micro-M range.