We used a dual-sampling chamber 2.5 cm in diameter, controlled by an Apple II microcomputer, to measure gas tensions in the skin of adults by mass spectrometry. The tensions of O2, CO2, N2, and water vapor and the resistances to the flows of gases through the skin were calculated from the voltages for the different gases passing through two separate membranes of different permeability. We compared the skin O2 and CO2 tensions with arterial values in 13 patients with lung disease during a standardized exercise test. The skin values followed changes in the arterial values. The regression line relating skin to arterial O2 tension had a slope of 1.05 and that for CO2 a slope of 1.51. We conclude that mass spectrometry is capable of providing measurements of gas tension from the heated skin that are similar to those found using electrochemical methods in adults.