Intranasal challenge of allergic subjects with the allergen to which they are sensitive rapidly produces sneezing, rhinorrhea, and airway obstruction. Nasal washings during the response reveal increased amounts of histamine, leukotrienes, prostaglandin D₂ (PgD₂), kinins and TAME-esterase in secretions. Although appearance of these mediators ceases shortly after challenge, many patients have a recrudescence of symptoms 3–11 h later, with a reappearance of the same mediators with the notable exception of PgD₂. Subjects who respond to exposure to cold with rhinorrhea and nasal stuffiness were subjected to a 15-min nasal challenge with cold (––3–10 °C) dry (10% relative humidity) air and also responded with typical symptoms and the appearance of histamine, PgD₂, TAME-esterase and leukotrienes. Nasal challenge with ragweed pollen by patients on immunotherapy showed that the threshold for response was greater and the amount of mediator found was less after treatment.