Since vascular architecture differs among tissues, it was hypothesized that the change in transverse relaxation rate produced by a given tissue concentration of susceptibility contrast agent also varies by tissue. This is relevant to strategies to map regional blood volume by MRI using indicator dilution techniques. R*(2) was measured in rat organs over a range of susceptibility agent concentrations at 1.5 T. Rat red blood cells loaded with dysprosium-DTPA-BMA served as an intravascular susceptibility agent. Tissue samples were frozen in vivo and dysprosium concentrations were independently measured using inductively coupled plasma atomic emission spectroscopy. The slope (k) of R*(2) vs. tissue dysprosium concentration in sec(-1) mM(-1) for myocardium was 97.1 (95% C.I. 77. 0-117.2), liver 122.6 (108.3-136.9), spleen 22.5 (8.8-36.3), kidney 68.1 (58.6-77.6), and skeletal muscle 77.9 (4.1-151.6); k was significantly different (P < 0.05) for all pairings except those with skeletal muscle. Therefore, relative values of tissue blood volume derived from dynamic images of first pass contrast effects may be in error because k is not constant for different conditions.