The transcutaneous oxygen tension was measured in 29 anaesthetized patients by means of a TCM1 TC Oxygen Monitor (radiometer). The correlation coefficient between these transcutaneously determined values and the true PaO2 values was 0,81 as compared with 0,93 in a previous trial in which a Servomed Oxymonitor (Hellige) was used. The effect of diathermy on the TCM1 TC Oxygen Monitor was much less than that on the Hellige apparatus.