The reproducibility of the changes in FEV, induced by inhalation of histamine, was studied in adult asthmatics. Histamine was inhaled from a Hudson nebulizer, delivering 0.3 ml/min, at tidal breathing through one minute. FEV was measured immediately before and two minutes after cessation of the inhalation. Identical concentrations of histamine were inhaled by the individual patient on every challenge. Reproducibility was studied on the same day, with one day inbetween the histamine challenges and with 12 weeks inbetween two challenges. When reproducibility during the same day was studied an average fall in FEV of 34% was found after the first inhalation and 34% decrease was found after the second challenge. When histamine challenge were done on two following days the fall in FEV was 25% on the first day and 28% on the second day. The average fall in FEV was 34% and 28% when reproducibility was studied with an interval of 12 weeks. No statistically significant difference was found between any of the compared challenges.