Methodical factors in the determination of bronchial responsiveness to histamine were studied in two groups of asthmatic children. In the first group, dose-response relationships of inhaled histamine and the provocation dose defined as the threshold (PD) were calculated by various methods, using different techniques of analysis of the dose-response relationship. These PD correlated strongly, but their absolute values of the dose-response relationship. These PD correlated strongly, but their absolute values varied. A 15% decrease in FEV1 was a suitable criterion for the PD (PD15). Other fixed criteria (PD20 and PD25) revealed PD values which had an arithmetical relationship with PD15 and produced no advantage. However, the PD values calculated on the basis of the intrasubject variability had a considerable variation with those using a fixed criterion. The response rate (slope of the dose-response curve) seems not to be related to the threshold. The time-response relationship was analysed in the second group. Bronchial obstruction after the inhalation of the PD and the preceding dose lasted for about 15 min. Nevertheless no cumulative effect was found giving sequential histamine concentrations inhaled at 3 min intervals.