Density is defined as mass per unit volume. The classical technique to measure the density of fluids consists of a determination of mass and volume. Blood density is proportional to hematocrit or, more exactly, to the total protein concentration of blood; only to a minor extent is blood density influenced by other plasma solutes. Since the introduction of the mechanical oscillator technique for the continuous recording of fluid density a sizeable amount of experience has accumulated. This review summarizes recent work performed with this technique. It appears that the scientific interest in a variable like blood density depends on the availability of a suitable and simple method. Until the oscillator technique was available the measurement of density was too complicated or too inaccurate for routine laboratory use. A further new technique permits us to determine fluid densities by measuring sound velocity transmission. The density dilution method can be used for the determination of distribution volumes, of flow through organs, and of the cardiac output. The influence of temperature and of certain artifacts like acceleration forces in the density measuring device have to be considered any may be used for additional diagnostic purposes like determination of erythrocyte sedimentation velocity. The new technique opens a reasonable simple way to study fluid shift between interstitial space and capillaries. The arterio-venous density gradient in an organ depends on the lymph production. The injection of a hypertonic solution leads to an osmotic fluid shift from the extravascular space towards the blood. This fluid shift can be recognized by a reduction of the blood density. A simple model for the description of this reaction is presented.