We studied changes in NAC in 17 ragweed-sensitive individuals after intranasal ragweed-challenge testing. All patients experienced immediate symptoms of sneezing, rhinorrhea, and nasal congestion that were associated with marked decreases in NAC (mean = 68%). In 10 trials patients also experienced late (greater than 0 hr) symptoms of nasal congestion with or without rhinorrhea; the mean late NAC decrease in this group was 42%. In contrast, no late symptoms were noted in nine trials, and the mean NAC decreased 5% in this group (p less than 0.003). Attempts to passively transfer immediate or late nasal sensitivity to one individual by spraying the nasal cavity with IgE antibody-containing serum, by packing the nose with cotton pledgets soaked in serum, by injecting serum directly into the inferior turbinate, and by transfusion with IgE-containing serum were not successful. We conclude that symptomatic late-phase reactions occur in the nose after intranasal challenge in about 50% of patients and that these symptomatic reactions can be confirmed objectively by rhinomanometry.