Abstract [ 153Gd]Gd(acetate) n was administered i.v. to mice to study the effect of dose on the distribution of free Gd. Distribution from blood was slow with the majority of the Gd distributing in the liver. Gd saturated in bone. Heart, lungs, kidneys, brain and skeletal muscle exhibited time-dependent decreases in Gd concentration. Gd that washed out of heart, lungs, kidneys and/or muscle redistributed in liver, spleen and femur. These results indicate a complex dose- and time-dependent tissue distribution for Gd and emphasize the importance of eliminating unchelated free Gd as a contaminant in Gd-chelates before testing in biodistribution experiments. The long-term residual accumulation of Gd suggests the need to minimize Gd-chelate dissociation in vivo.