[35S]L-Homocysteine thiolactone ([35S]L-HCTL) was synthesized and its biodistribution evaluated as a potential brain radioprotective agent and as a tissue hypoxia marker. Drug uptake in mouse brain exceeded that in s.c. tumor 3 h post injection only. Multiple indicator dilution experiments in the rabbit heart indicate that membrane permeability of [35S]L-HCTL does not limit its usefulness as a hypoxia marker. In addition, a positive correlation was observed between regional coronary blood flow and myocardial content of [35S]adenosylhomocysteine formed from [35S]homocysteine and adenosine.