Background: The level of histamine in nasal lavage fluid has been used as an index of mast cell/basophil activation in a number of studies. Obviously, such an index can only be valid if changes in the secretory activity of nasal glands do not affect the level of histamine in lavage fluid (i.e. hypersecretion, without a simultaneous activation of mast cells/basophils in the nasal mucosa, must not increase the level of histamine). Objectives: To asses the effect of nasal hypersecretion on histamine levels in lavage fluid. Methods Nasal challenges were performed with methacholine and allergen in grass pollen-allergic patients and non- allergic controls. Nasal lavage fluid was collected before and repeatedly for nine hours after nasal challenge, and the level of histamine was compared with that of a specific mast cell-derived enzyme, tryptase. In addition, the effect of methacholine on basophils was examined in vitro. Results: Allergen challenge of allergic patients produced sneezing and a significant increase in histamine and tryptase levels, whereas challenge of non-allergic subjects produced no such response. Interestingly, challenge with methacholine also induced a significant increase in histamine levels. This increase was seen in both allergic and nonallergic subjects and it was not associated with any sneezing or increase in tryptase levels, indicating that mast cells were not activated. Furthermore, stimulation of basophils with methacholine did not induce any histamine release in vitro. Conclusions: Apparently, there exists a pool of histamine in the human nose that can be transferred to lavage fluid during glandular hypersecretion. The source of this histamine is yet to be identified. As the level of histamine seems to be affected by the secretory activity of nasal glands, we question the use of this single mediator as an index of mast cell/basophil activation in nasal lavage studies.