We studied the effect of alpha-fluoromethyl histidine, an irreversible histamine synthesis inhibitor, on the immediate nasal reaction to antigen challenge in a double-blind, placebo controlled, randomized, parallel study using 13 subjects. The patients received either active drug 100 mg twice daily or placebo, for 3 weeks. A nasal allergen challenge was performed before and after at weekly intervals. Symptoms at challenge were assessed and the levels of histamine, TAME-esterase activity and kinins were measured in nasal lavages before and after antigen challenge. Skin tests were also performed at weekly intervals. In addition, the urinary excretion of the main histamine metabolite, telemethylimidazole acetic acid, was measured before and after 3 weeks of treatment. The active treatment induced 60% reduction in histamine levels in the lavage fluids before and after antigen challenge, as well as a reduction in the histamine levels in the lavage fluids before and after antigen challenge, as well as a reduction in the main urinary histamine metabolite. However, no reduction was found in nasal symptoms obtained after antigen challenge. The levels of kinins and TAME-esterase activity were not significantly reduced.