Standardization of bronchial challenge (BC) in order to improve repeatability of both non-specific and specific challenges is desirable. In the tidal volume breathing BC (tBC), the patient inhales bronchoconstrictor during the entire inspiratory phase of the tidal volume breathing. We determined 2 min ventilation (VE) at each histamine concentration during 187 BCs. Thirty-four patients were challenged twice. We used a standard technique (Cockcroft), except for our patient operated Pari nebulizer. Between-subject (SD = 37%) as well as within-subject (SD = 21%) (p less than 0.01) variations in VE were observed. Furthermore, VE during inhalation of threshold dose (VE-threshold) was smaller than VE during inhalation of the previous dose (VE-threshold-1) (p less than 0.001). For paired differences in VE during two challenges, the within subject SD was 21%. The between subject SD for VE was 32%. VE-threshold was lower in first challenge (p less than 0.001). VE threshold in both challenges was significantly lower than VE-threshold-1. These observations indicate that the respiratory pattern during tBC needs further standardization and that respiratory frequency and inspiratory time during tBC would be worth investigating.