With the forced pseudo-random noise oscillation technique (FOT), resistance (Rrs) and reactance (Xrs) of the respiratory system can be measured simultaneously over a frequency spectrum of 2-26 Hz. As only passive cooperation of the child is needed, FOT is suitable for lung function measurements from the age of 2 1/2 years. Hence bronchial responsiveness can be measured in children who are not yet able to perform spirometry or flow-volume curves. We compared bronchial responsiveness to histamine and methacholine obtained with FOT. Threshold dose or provocative dose to histamine and methacholine showed a close correlation in asthmatic children aged 3.6 to 7.8 years. The 24 hour interval within-subject reproducibility of threshold dose and provocative dose to histamine in asthmatic children aged 3.9 to 8.5 years proved to be good. Bronchial responsiveness to histamine or methacholine measured by FOT was not influenced by baseline lung function or by bronchial smooth muscle tone.