The MDO (Mehrdraht Dortmund oberfläche) oxygen electrode was used in a study of skeletal muscle oxygen pressure fields, presented as histograms, in critically ill patients artificially ventilated with gas mixtures of different oxygen concentrations. The histograms were compared with forearm blood flow measurements performed with strain gauge plethysmography. Local blood flow and permeability-surface area product (PS) were also studied by the simultaneous clearances of 133xenon and 51Cr-EDTA. The histogram distribution type was normal, i.e. approximately Gaussian, at arterial oxygen pressure levels between 10 and 18 kPa. At arterial oxygen pressures outside this range the histogram distribution types were abnormal, i.e. they showed a non-symmetrical distribution of oxygen pressure values, but their mean was approximately the same as in the normal histogram. However, there were significantly higher tissue oxygen pressure mean values in the patients (3.43 kPa) than in a group of healthy human volunteers (2.25 kPa). Mean forearm blood flow and the clearances of 133xenon and 51Cr-EDTA showed marked variations during the measurements both intraindividually and interindividually. Mean forearm blood flow and mean clearances of 133xenon showed opposite trends compared with arterial oxygen pressures. Mean clearances of 51Cr--EDTA and means PS showed only minor variations at the different arterial oxygen pressure levels.