A technique has been developed forin vivodetermination of lithium content in the head (and potentially in the whole body and in selected organs) of patients undergoing lithium therapy. It is based on the measurement of tritium induced by the6Li(n,α)T nuclear reaction after neutron irradiation of the body. The fraction of tritium exhaled in the expired air in the form of HT is collected, separated from the other gases, and counted in a high‐sensitivity beta counter. The feasibility of the technique was demonstrated by measurements of lithium in the head of a sheep and in the whole body of rats, following the administration of  6LiCl (enriched6Li isotope, 95.46% abundancy). The precision of the technique is acceptable for clinical applications based on a maximum propagated error of 8.4%. The sensitivity is 1 count/d (from T activity) per 10 mSv (total dose) and 1 μg of6Li. This indicates that studies on patients under6Li treatment can be performed successfully with a radiation dose to the head of 1–2 mSv.