The purpose of this study in rodents was to determine the pathologic basis for enhancement patterns of hyperplastic and tumours lymph nodes shown by MR lymphography after IV injection of superparamagnetic iron oxide nanoparticles (AMI-227). Hyperplastic and tumorous lymph nodes were imaged in vivo at 1.5 T 1 day after IV administration of AMI-227 (40-200 mumol Fe/kg) to rats. Inguinal and axillary lymph nodes were surgically resected, and in vitro imaging was done by using the same magnet with a prototype coil that allowed a pixel size of 70 x 230 microns. Imaging findings were correlated with histologic findings and iron staining of the nodes. With all sequences, at doses of 80 mumol Fe/kg or higher, the signal intensity of hyperplastic nodes was lower than on unenhanced images. The same pattern was seen in the remaining normal tissue of tumourous lymph nodes. This T2* effect was a result of clustered particles inside macrophages in the lymphatic sinuses. At doses of 200 mumol/kg, the tumor itself consistently showed an increase in signal intensity on T1-weighted images. This T1 effect probably resulted from particles leaking into the interstitial spaces of the tumor. After IV injection of superparamagnetic nanoparticles, a decrease in signal intensity indicates active uptake of particles into macrophages, whereas an increase in signal intensity indicates altered capillary permeability in tumor. These findings in rats may prove to be clinically useful in the future for differentiating benign from malignant enlarged lymph nodes.