An elevated concentration of nitric oxide (NO) in alveolar ventilation is indicative of inflammatory stress within the lung. We present here the first description of time-resolved measurement of NO in breath using photoionization mass spectrometry, providing new capabilities for the medical investigator, such as isotopic tracing. Here we use resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry (TOF-MS) coupled with a medium pressure laser ionization (MPLI) source for the selective detection of NO in breath. To demonstrate this technology, a single male subject breathes NO-free air for several minutes, and then the exhaled breath is monitored. The ability of REMPI to differentiate among three different isotopomers of NO is demonstrated, and then the concentration profile of NO in exhaled breath is measured. A similar time-dependence concentration is found as observed by previous techniques. The advantages of this approach compared to other techniques are: (1) parts-per-billion by volume (ppbV) mixing ratios of NO can be measured on a sub-second time scale, (2) since the technique operates optically as well as mass-resolved, isotopomers of NO are discernable, permitting the use of isotopic tracing, and (3) other biologically significant gas molecules can be measured via REMPI.