Adsorption and electrode reactions of oxytocin on a mercury electrode have been investigated by d.c. polarography, a.c. polarography and cyclic voltammetry. The first monolayer of oxytocin adsorbed tended to undergo a reversible electrode reaction, while the subsequent layers were reduced by means of an irreversible electrode process. The area occupied by an oxytocin molecule in the first monolayer varied between 135 Å2 and about 300 Å2 depending on adsorption potential and other conditions. The weak hydrophobic adsorption of the polypeptide is enhanced by the formation of mercury sulfide bridges at potentials more positive than the reduction half wave potential. Desorption peaks are obtained in a.c. and oscillo-polarograms at potentials more negative than the half wave potentials after the mercury sulfide bridges have been reduced.