A double-blind, placebo-controlled clinical trial was performed on 12 subjects with allergic rhinitis to evaluate the effect of 1-wk pretreatment with terfenadine (60 mg twice daily or 300 mg twice daily) on the response to sequential nasal challenges with allergen and histamine. The response to allergen challenge was monitored by counting sneezes and by quantifying the levels of histamine, kinins, TAME-esterase activity, and albumin in recovered nasal lavages. Following histamine challenges, sneezing and the levels of albumin were recorded. Terfenadine significantly reduced sneezing and the levels of histamine, kinins, TAME-esterase activity, and albumin seen after antigen challenge. In addition, terfenadine reduced the increase in sneezing and vascular permeability, as measured by albumin levels, following challenge with histamine. No significant differences were found between the two doses of terfenadine. Our data suggest that terfenadine not only antagonizes the H1 activity of released histamine during the immediate allergic reaction but also, at currently prescribed doses, inhibits the release of histamine.