The identification of apometallothionein (AMT) in rat liver by reversed-phase high-performance liquid chromatography (RP-HPLC) after gel permeation was realized in experiments performed both in vivo and in vitro. The reliable assignment of the corresponding AMT peak permitted the detection and determination of AMT in different groups of experimental and control rats. In all animals studied (more than 100 rats), AMT was always present in amounts higher than that of metallothionein (MT) or compatible with it. Induction of MT synthesis by CdCl2 subcutaneous injections decreased the AMT level and increased the MT level, but nevertheless the amount of AMT still remained relatively high.