Since the absolute density of dopamine receptors can vary in disease, it is essential to establish the normal values for the absolute densities of D1 and D2 receptors in the brain. Absolute densities are most conveniently reported in units of picomoles per gram of original tissue, readily permitting their comparison to data obtained by positron emission tomography in patients. The density of D1 receptors is approximately 120 pmol/g in the rat striatum and 19 pmol/g in the human striatum. The density of D2 receptors is about 32 pmol/g in the rat striatum and 14 pmol/g in the human striatum, these values being determined by Teflon-glass homogenization and the centrifugation method. The customary Polytron-homogenization procedure results in a loss of about 9% of the D2 receptors in rat tissue and about 28% in human tissues; filtration results in a further loss of about 12%. There is general agreement between the in vitro and in vivo densities, but only if the receptors are measured by the amount of radioisotope specifically displaced.