Noninvasive blood gas monitoring is a new, simple, and reliable method for assessing hyperreactivity associated with bronchial asthma. In this study, 104 atopic rhinitic subjects with no history of wheezing and 104 healthy volunteers were challenged with ultrasonically nebulized distilled water (UNDW). Blood gases were monitored transcutaneously (PtcO2 and PtcCO2) over 42 min (20 min for electrode stabilization, 3 min for monitoring a steady baseline, 5 min during UNDW, and 14 min after UNDW). Mean baseline PtcO2 and PtcCO2 values were comparable in the two groups. In rhinitic subjects only, a sudden decrease in PtcCO2 (starting immediately after the beginning of the challenge and maximal 34.7 +/- 0.4 mm Hg SEM versus baseline 41.8 +/- 0.2 SEM mm Hg at the third minute of UNDW exposure) was induced by the challenge and proved significant (p less than 0.001). In the same subjects, a slightly delayed decrease in PtcO2 (starting immediately after the end of UNDW inhalation and maximal 64.5 +/- 1.1 mm Hg SEM versus baseline 78.3 +/- 0.7 SEM mm Hg at 4 min post-UNDW) was also induced by the challenge and proved highly significant (p less than 0.001). The effects of UNDW inhalation on blood gases in normal subjects were negligible and nonsignificant. UNDW in nonasthmatic rhinitis but not in normal subjects gives rise to a sudden hyperventilation and to gas-exchange abnormalities presumably reflecting a ventilation-perfusion mismatching, which, however, is of shorter duration in rhinitic than in asthmatic subjects.