Mössbauer spectroscopy was used to investigate the distribution of iron in rat organs and its localisation in liver subcellular fraction. A 57Fe-sucrose complex solution was injected by 0.5 ml doses into tail veins of animals every day, during a 6-day period. Mössbauer spectra were measured in spleen, blood, liver and liver subcellular fractions. The mössbauer spectrum of a spleen sample has two symmetrical doublets, one with delta = 0.6 mm/s and delta = 0.7 mm/s, and the other with delta = 1.0 mm/s and delta = 2.35 mm/s. The Mössbauer spectrum of blood has parameters which are close to those for carboxyhemoglobin and oxyhemoglobin complexes. After the addition of sodium citrate, the proportion of the carboxyhemoglobin complex increases. The Mössbauer spectrum of liver has a two-component pattern with two symmetrical doublets, the first with delta = 0.6 mm/s and delta = 0.63 mm/s and the second with delta = 1.4 mm/s and delta = 3.45 mm/s. The first component, which was identified as ferritin, is present in all subcellular fractions (800 x gav sediment fraction, mitochondrial/lysosomal, microsomal and supernatant fractions), with its greatest content in microsomal fraction. After the addition of NaBH4 to mitochondrial/lysosomal fraction, about 20% of the iron contained in ferritin was reduced. In the Mössbauer spectrum this is reflected by an appearance of a doublet with delta = 0.85 mm/s and delta = 3.7 mm/s.