Four novel macromolecular MRI contrast agents, all of which had the same chemical composition but different molecular weights, were prepared using generation-3, -4, -5, and -6 poly-amidoamine (PAMAM™) dendrimers conjugated with a bifunctional diethylenetriaminepentaacetic acid derivative to change the blood retention, tissue perfusion, and excretion. Size-dependent changes in the pharmacokinetics were observed in the biodistribution study. 153Gd-labeled generation-6 PAMAM™conjugates remained in the blood significantly longer than all of the other preparations (P &lt
0.001). The increase in blood-to-organ ratio of the preparations was found to correlate with increasing molecular size (P &lt
0.001). Additionally, 3D-micro MR images and angiography of mice of high quality and detail were obtained using PAMAM™-(1B4M-Gd)× as a macro-molecular MRI contrast agent with a 1.5-T clinical MRI instrument. Numerous fine vessels of ∼200 μm diameter were visualized on subtracted 3D-MR angiographs with G6D-(1B4M-Gd)192. The quality of the images was sufficient to estimate the microvasculature of cancerous tissue for anti-angiogenesis therapy and to investigate knockout mice. © 2001 Wiley-Liss, Inc.