Abstract Iron compartments in which iron accumulates during the inhibited heme synthesis after treatment with isonicotinic acid hydrazide were studied in rabbit reticulocytes. A great accumulation of 59Fe radioactivity was found in mitochondria, low molecular weight iron compounds and non-hemoglobin proteins, especially ferritin. In a chase experiment, approximately 50% of the 59Fe radioactivity accumulated in mitochondria and low molecular weight iron compounds was re-utilized for the synthesis of hemoglobin. Although some iron is incorporated into ferritin, it apparently is not utilized for heme synthesis. In control reticulocytes, only traces of 59Fe radioactivity were detected in mitochondria and only a minute amount of 59Fe radioactivity was detected in low molecular weight iron compounds. The release of low molecular weight iron from 59Fe-transferrin occured in intact reticulocytes to a larger extent than in the stroma-free hemolysate. An attempt to establish the possible pathway of iron transport inside the reticulocyte was made. It is suggested that an iron-transferrin complexes enters the reticulocyte cytoplasm, and the majority of released iron is taken up by mitochondria for heme synthesis. When protoporphyrin IX is not available, iron accumulates inside the mitochondria.