The effects of halothane and enflurane on the polarographic measurement of oxygen with five platinum and three gold micro-electrodes were examined. Oxygen microelectrodes were calibrated in saline solution equilibrated with either nitrogen (N2) or air, then either halothane, 1.0 per cent, or enflurane, 2.0 per cent, was added to the gas mixture. For each electrode, polarographic curves were determined during exposure to five equilibrating gas mixtures: N2, air, N2 plus halothane, air plus halothane, and N2 plus enflurane. Halothane variably increased the current produced (and therefore the estimated oxygen tension) at all polarizing voltages in saline solution equilibrated with either N2 or air. The effect was present in both conical platinum electrodes and recessed-tip gold electrodes and was not prevented by membrane coatings of polystyrol, Rhoplex or collodion. Enflurane did not alter the polarographic measurement of oxygen. It is concluded that tissue oxygen tension measurements, made with these microelectrodes and membranes, may be unreliable in the presence of halothane.