Metallothioneins (MTs) belong to a group of oligo- and polypeptides which play an important role in the metabolism of metals in animals, plants and microorganisms. The metabolic function of MTs consists not only in detoxication of an organism but also in the homeostasis of essential metals. This paper reports a novel approach to electroanalysis of MTs. The electrochemical determination of MTs is based on catalytic processes which proceed at very negative potentials on mercury electrodes (from -1.7 V to -1.9 V vs. Ag/AgCl/KCl). These processes accompanied by evolution of hydrogen from supporting electrolyte components, include the Heyrovsky presodium wave and/or Brdicka reaction. It was found that SH groups present in MTs are responsible for catalytic processes. The catalytic signal of a MT at nanomolar concentrations can be detected on mercury electrodes using potentiostatic electrochemical methods. The highest sensitivity in the determination of MTs was observed with a galvanostatic method such as derivative potentiometric stripping analysis (CPSA) which produces the peak H. The coupling of CPSA with the adsorptive transfer stripping technique (AdTS) allows the determination of MTs at a femtomole level in a low amount of sample (5 ěl). Our recent results obtained by AdTS + CPSA show that detection limits for particular MTs can be improved by adding [Co(NH3)6] Cl3 to the sample analyzed. This is probably due to the formation of a complex between [Co(NH3)6] Cl3 and MT. We think that the peak H can be used in physiological studies of metal metabolism.