A submersible probe with a flow-through cell allowing in situ voltammetric measurements by means of either a mercury film or a mercury drop electrode has been developed. The various possibilities of voltammetric measurements in the presence of dissolved oxygen have been reviewed and tested. Optimum conditions for the determination of trace metals in oxygenated waters have been found. In situ determinations of Mn(II) in anoxic lake water and of trace metals (Cu 2+, Pb 2+, Cd 2+, Zn 2+) in oxygen-saturated sea water are reported.