Rat liver metallothioneins (MTs) were studied by means of differential pulse polarography (DPP) and preparative electrolyses in aqueous buffered phosphate solutions (0.05 moll(-1)) at micromolar concentrations. DP polarograms of MTs involving cadmium and zinc, (Cd,Zn)MT, showed four peaks at approximately -0.45 (A), -0.55 (C), -0.85 (B) and -1.1 V (D) vs, SCE. Remetallation reactions of the apoprotein, thionein, allowed peaks A and B to be attributed to the two-step reduction of cadmium, and peaks C and D to the two-step reduction of zinc centres. Copper in MTs showed two DPP peaks only in the presence of EDTA (at -0.35 and -0.75 V). The two-step one-electron reduction of bound metals (proved by coulometry for the case of cadmium thionein) indicated the metal dithiolene character of the metal core in MTs.