We report a new method for examination of bronchial reactivity by measuring transcutaneous oxygen pressure (tcPO2) during the inhalation of histamine in stepwise incremental concentrations. The correlation between changes in tcPO2 and those in PEFR or FEV1 was high (P less than .001). When the fall in PEFR or FEV1 was more than 20% compared with baseline, the fall in tcPO2 was more than 10%. We also measured the dose of radiolabeled aerosol (99mTc-DTPA) inhaled into the lung using a scintillation camera. When aerosol is inhaled during tidal breathing, there are adequate correlations between the dose of aerosol in the lung and both the age and height of the patients (P less than .001). When aerosol was inhaled during crying, little deposit within the lung was shown. When the inhalation challenge test was done during a sleeping period for children aged under 2 years, the loading dose of aerosol in the lung was sufficient. The geometrical means of respiratory threshold of histamine (RT-Hist) among 106 asthmatic and 11 control children, aged 2 to 6 years, were 1182 and 4414 micrograms/mL, respectively. The means of RT-Hist in 17 children with bronchial asthma, nine with atopic dermatitis, five with respiratory disease, and nine controls, aged under 2 years, were 1152, 964, 544, and 3402 micrograms/mL, respectively.