Strong vertical mixing during winter in the northwestern Mediterranean Sea occurs with simultaneous ventilation of water masses. In the high turbulent deep water formation chimneys, with homogenous density from the sea surface to the bottom, the oxygen input, limited to the most intense winter period, reaches almost 10 super(3) l O sub(2) m super(-2). From Redfield's exchange coefficient, an oxygen flux of about 12 l m super(-2) d super(-1) can be calculated. Nearly the same flux can be derived from Broecker and Peng's piston velocity data, but in order to reach this value one has to assume that the sea surface film thickness averages 10 mu m. Biological oxygen fluxes produced by photosynthesis or respiration are almost negligible in comparison with the atmospheric oxygen input. But this is not the case in the frontal zones surrounding the turbulent mixing zone.