Cutaneous O2-uptake has been estimated in plaice, Pleuronectes platessa, naturally buried in sediment as the difference between total O2-uptake, measured in a flow-through respirometer, and branchial O2-uptake calculated from direct and continuous recordings of gill water flow and O2-extraction from the ventilatory current. At conditions of aerated water cutaneous O2-uptake made up 27% of total uptake. During exposure to hypoxic water, cutaneous O2-uptake declined less than O2-uptake across the gills. Hence the relative importance of cutaneous O2-uptake increased with progressive hypoxia, making up 37% of total at a water PO2 of 40 mm Hg. Factors are discussed which may affect the level of cutaneous O2-uptake at changing conditions of ambient O2-availability.