To evaluate the relationship between dynamic contrast agent–enhanced magnetic resonance (MR) imaging–derived kinetic parameters and contrast agents of equal chemical composition and configuration but with different molecular weights in a tumor angiogenesis model. This study was approved by the ethical review committee. Maintenance and care of animals was in compliance with guidelines set by the institutional animal care committee. Dynamic contrast-enhanced MR imaging was performed with dendritic contrast agents in 16 mice with tumor xenografts; mice were placed in groups of four for each molecular weight of the contrast agent. The magnitude and spatial distribution of kinetic parameters (transfer coefficient [KPS] and plasma fraction [fPV]) were compared with molecular weight of the contrast agent by determining the Spearman correlation coefficient (r) and the quantitative relationship between the endothelial KPS and molecular weight. Inverse relationships between molecular weight of contrast agent and KPS and fPV of tumor rim (r = –0.8, P <.001 and r = –0.5, P = .04, respectively) and core (r = –0.7, P = .004 and r = –0.6, P = .01, respectively) were observed. The quantitative relationship between KPS and molecular weight (MW) was KPS = 0.4/MW0.44. A decreasing stepwise pattern in fPV was noted between contrast agents with low (0.7- and 3.0-kDa) molecular weight and those with high (12- and 51-kDa) molecular weight. Macromolecular permeability is best measured with high-molecular-weight contrast agents; endothelial KPS values measured with low-molecular-weight contrast agents incorporate tissue perfusion and permeability and demonstrate heterogeneous microcirculatory flow.