Summary.We have investigated the relation between the ripening rate of the reticulocytes and the respiration of the blood cells.1). The ripening rate can be increased several times by means of ripening substances without any increase in the oxygen uptake of the cells.2). After some hours' respiration the rate of oxygen uptake is considerably decreased, while the rate of the ripening is unaltered.3). Reduced oxygen pressure reduces both the oxygen uptake and the ripening rate.4). Drugs with influence on the respiration have a similar. influence on the ripening rate. Narcotics, malonate, cyanides and carbon monoxide give a decreased oxygen uptake and a decreased ripening rate, while the addition of fumarate and oxalacetate increase both.5). The oxygen uptake of the blood cells can he arrested for a few minutes by the addition of oxalacetate.6). The non‐reticulated red blood cells have some respiration too.7). The results seem to indicate that the ripening of the reticulocytes is linked to a part of the respiratory processes of the cells, but the greater part of the respiratory processes has nothing, to do with the development of the un