Biomedical imaging is valuable for non-invasive investigation of in vivo drug delivery with polymer conjugates. It can provide real-time information on pharmacokinetics, biodistribution and drug delivery efficiency of the conjugates. Non-invasive visualization of in vivo drug delivery of polymer conjugates with contrast enhanced magnetic resonance imaging (MRI) was studied with paramagnetically labeled poly(L-glutamic acid) in an animal tumor model. Poly(L-glutamic acid) is a biocompatible and biodegradable drug carrier for diagnostics and therapeutics. Poly(L-glutamic acid)-1,6-hexanediamine-(Gd-DO3A) conjugates with molecular weights of 87, 50 and 28 KDa and narrow molecular weight distributions were prepared and studied in mice bearing MDA-MB-231 human breast cancer xenografts. Contrast enhanced MRI resulted in real-time and three-dimensional visualization of blood circulation, pharmacokinetics, biodistribution and tumor accumulation of the conjugates, and the size effect on these pharmaceutics properties. The conjugate of 28 KDa rapidly cleared from the circulation and had a relatively lower tumor accumulation. The conjugates with higher molecular weights exhibited a more prolonged blood circulation and higher tumor accumulation. The difference between the conjugates of 87 and 50 KDa was not significant. Contrast enhanced MRI is effective for non-invasive real-time visualization of in vivo drug delivery of paramagnetically labeled polymer conjugates.