Physiological leucocytosis refers to an increase in the number of circulating leucocytes which occurs either in response to physical exercise or to increased levels of adrenalin in otherwise healthy human beings. The mechanism underlying this phenomenon and its physiological implications are not known. The article briefly sums up 99 years of research in this field. It is concluded that adrenalin is the most important mediator in the regulation of the leucocytosis. Before entering the circulating blood the leucocytes are stored intra- and/or extra-vascularly in several organs, including the lungs, spleen, lymphatic tissue and haematopoietic bone marrow. Evidence is also presented indicating changed activity at the level of the individual leucocyte during physiological leucocytosis.