Because of the growing interest in 131Cs as a tracer for myocardium and thyroid scintigraphy, its internal disometry in man was reviewed and determined. The absorbed dose was calculated using the method of Loevinger and Berman, with minor modifications, i.e. taking into account the different activities of the body organs and their contribution to the absorbed dose. Special attention was also paid to the softer and non-penetrating radiations, namely L and M, capture events and Auger electrons, in relation to the decay of the nuclide. Dosimetric data for the thyroid were determined experimentally.