During both animal and clinical studies with manganese (II) N,N'-dipyridoxylethylenediamine-N,N'-diacetate 5,5'-bis(phosphate) (DPDP), enhanced rims around liver tumors on magnetic resonance (MR) images have been observed. To elucidate the origins of these rims and to assess their potential value in the differential diagnosis of liver masses, the authors studied 15 rats with induced hepatocellular carcinoma (HCC), 10 rats with implanted Novikoff hepatomas, and 11 rabbits with implanted VX2 carcinomas. A total of 69 primary and secondary liver cancers from these three animal models were studied. Mn-DPDP- and gadolinium tetraazacyclododecantetra acetic acid (DOTA)-enhanced MR images were compared. On the Mn-DPDP-enhanced images, 34 peritumoral rims of various patterns were displayed, all of which were exclusively related to the presence of highly malignant primary and secondary liver tumors. Peritumoral zones of malignant infiltration, surrounding parenchymal compression, and bile duct proliferation were seen to be the origin of these rims after comparison of MR images with the corresponding microangiograms and histologic specimens.