This invention relates to a pharmaceutical composition for parenteral or oral administration containing a radioactive compound which can be used diagnostically or therapeutically. The composition comprises a micro-emulsion constituted by a dispersion of vesicles or microsponges of a fatty acid based component in an aqueous or other pharmacologically acceptable carrier in which nitrous oxide is dissolved, the fatty acid based component comprising at least one long chain fatty acid based substance selected from the group consisting of free fatty acids and derivatives of free fatty acids, and the radioactive compound.