Tritium-labelled dihydro derivatives of the cyanobacterial peptide hepatotoxin nodularin were prepared by reduction with sodium boro[3H]hydride. The optimised reaction gave two dihydronodularin stereoisomers which were purified by high-performance liquid chromatography with a mobile phase of methanol-0.7% sodium sulfate (6:4) and a C18 stationary phase. The specific activities of the stereoisomers were 1780-1807 dis min-1 ng-1. The radiolabelled dihydronodularins were tested for stability and used for toxicokinetic studies in mice. Liver was the main site of toxin accumulation.