Our present work is aimed at obtaining information about noradrenaline storage particles in splenic nerve—both of axonal and terminal derivation. These experiments have been carried out using differential and density gradient centrifugation techniques. Because of difficulties in obtaining an absolutely pure preparation of noradrenaline vesicles (Stjärne 1966) we have not attempted a purification of the particles but have used the other possible alternative as first proposed by de Duve (1964). This method involves the localization of a particle, in fractions obtained by centrifugation, by estimation of an internal constituent; the degree of contamination of a particulate fraction can be assessed by estimation of constituents of other known particles. By this method it is then possible to determine which constituents belong to any one particle.