{Fe[Gd2bpy(DTTA)2(H2O)4]3}4- is a self-assembled, metallostar-structured potential MRI contrast agent, with six efficiently relaxing Gd3+ centers confined into a small mol. space. Its proton relaxivity is particularly remarkable at very high magnetic fields (r1 = 15.8 mM-1 s-1 at 200 MHz, 37°C, in H2O). Here we report the first in vivo MRI feasibility study, complemented with dynamic g scintigraphic imaging and biodistribution expts. using the 153Sm-enriched compd. Comparative MRI studies have been performed at 4.7 T in mice with the metallostar and the small mol. wt. contrast agent gadolinium(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate ([Gd(DOTA)(H2O)]- = GdDOTA). The metallostar was well tolerated by the animals at the concns. of 0.0500 (high dose) and 0.0125 (low dose) mmol Gd kg-1 body wt.; (BW). The signal enhancement in the inversion recovery fast low angle shot (IR FLASH) images after the high-dose metallostar injection was considerably higher than after GdDOTA injection (0.1 mmol Gd kg-1 BW), despite the higher dose of the latter. The high-dose metallostar injection resulted in a greater drop in the spin-lattice relaxation time (T1), as calcd. from the inversion recovery true fast imaging with steady-state precession (IR TrueFISP) data for various tissues, than the GdDOTA or the low dose metallostar injection. In summary, these studies have confirmed that the approx. four times higher relaxivity measured in vitro for the metallostar is retained under in vivo conditions. The pharmacokinetics of the metallostar was found to be similar to that of GdDOTA, involving fast renal clearance, a leakage to the extracellular space in the muscle tissue and no leakage to the brain. As expected on the basis of its moderate mol. wt., the metallostar does not function as a blood pool agent. The dynamic g scintigraphic studies performed in Wistar rats with the metallostar compd. having 153Sm enrichment also proved the renal elimination pathway. The biodistribution expts. are in full accordance with the MR and scintigraphic imaging. At 15 min post-injection the activity is primarily localized in the urine, while at 24 h post-injection almost all radioactivity is cleared from tissues and organs.