To know where given amounts of blood might be, we need to know the potential vascular space and what the distribution among various sized vessels is. Methyl methacrylate was injected via the cannulated aorta into 4 rats, and vascular casts made. Polymerization caused 15 per cent shrinkage. The casts were separated into 3 compartments: 150 μ. and smaller, 150 to 600 μ., and 600 μ. and larger. Using the shrinkage, specific density, and weights of plastic, the “total vascular spaces” in cc./100 Gm. were found to be 8.9, 10.6, 11.1 and 11.1, respectively. The mean percentages for the 3 compartments were 10.4 in vessels 706 μ and larger, 6.7 in 188 to 706 μ., and 82.9 in vessels 188 μ. and smaller. Because the only observed distortions of the casts on the basis of comparisons with the in vivo vasculature of a number of organs were some noninjected capillaries and nonmaximally injected veins, the approximations of “total vascular space” are considered too low. The utility of methyl methacrylate casts for gravimetric determinations of volumes was demonstrated. The finding of over 80 per cent of the vascular space in vessels 188 μ. and smaller is relevant to the potential location of blood within the body.