Abstract: The lithium used clinically and distributed in organs such as the brain or idney of humans and other exhaling animals is determined in-vivo by means of neutron radiation and measuring in the exhaled air elemental tritiated hydrogen released from the tritium reaction by the reaction .sup.6 Li(n,.alpha.)T. The tritium atoms so released are transformed in part in the surrounding aqueous solution to form gaseous tritiated hydrogen which has a small solubility in body tissues and liquids and thus appears quickly in the breath. After a recipient fasts and is irradiated with neutrons, the air exhaled in the breath for a given time after irradiation is captured and processed to remove water, isolate hydrogen and measure the tritiated hydrogen with a gaseous organ-methane counter.