A new, commercially available, transcutaneous (tc) PO2 monitor was tested in adult females and in laboratory animals to assess its applicability in measuring arterial oxygen tension during physiological stress. Observed values on dogs correlated well with direct measurements of arterial PO2 and with previous data obtained from measurements of arterial blood during exercise and hypoxemia. In our female subjects the unit responded rapidly to changes in inspired ambient oxygen and electrical stability was excellent during maximal exercise tests. Transcutaneous PO2 decreased to an average of 87.8 Torr during maximum exercise breathing 20.9% O2, and to 32 Torr while breathing 12.6% O2 at maximum work. Two distinct patterns of response to tc PO2 were observed during hypoxic and normoxic exercise. The technique appears to have substantial future application both in clinical and physiological investigation involving adult subjects.