Ex vivo freeze-dried rat muscle tissues, collected at different times t after a single dose of subcutaneously injected iron dextran, have been magnetically characterised. The AC susceptibility of the tissues shows an overall superparamagnetic behaviour and the dependence on t of, especially, the out-of-phase component is remarkably systematic despite the fact that each tissue originates in a different rat individual. The experiments show that the akaganéite (beta-FeOOH) nanoparticles contained in the injected drug are progressively degraded in the living tissue and, at times of the order of 1 month and for all the analysed rat individuals, converge to a magnetically well-defined species with much narrower magnetic activation energy distribution than iron dextran. Thorough transmission electron microscopy experiments of the same tissues indicate the presence of oxyhydroxide particles, whose size decreases for increasing t in agreement with the interpretation of the magnetic susceptibility. The conclusions drawn from the magnetic study do well correspond to the properties of the whole tissue since no biochemical extraction work has been done. The AC susceptibility appears to be a valuable and complementary tool in pharmacological studies of iron-containing drugs.