Abstract: Apparatus for the determination of pO.sub.2 -histograms by means of a needle probe.The apparatus for the determination of pO.sub.2 -histograms in living tissue comprises a macro needle probe of larger diameter moved stepwise between individual measurement locations, the step widths being in the order of magnitude of the probe diameter and the speed being considerably higher than pO.sub.2 -changes within the tissue. Measured values are determined shortly after the probe has come to a standstill. Thus, disturbances in the tissue pO.sub.2 due to compression effects induced by the thick probe do not falsify the measured value although a thick and stable probe is used. A polarographic measuring site with a ring-shaped metal film electrode used here is characterized especially by a recessed metal ring electrode. By this means a measuring site of low stirring effect and very short response time is obtained.