A simplified method for the extraction and isolation of histamine from blood, plasma and freeze-dried tissue is described. Extraction is carried out with absolute or 80% ethanol. Subsequently histamine is isolated from an ion-exchange column and determined fluorometrically. The normal histamine contents of blood, plasma and various organs from guinea pigs are investigated by this procedure. The values obtained are compared with the data in the literature. On comparison of the histamine contents in the organs of guinea pigs of two weight and age classes, significant differences are found in the lungs, heart, spleen, kidneys and stomach. There seems to be an inverse relationship between body weight and histamine concentration.