The change of ion transport in acute allergic reactions in vivo was studied by measuring the nasal transepithelial potential difference (PD) in patients with nasal allergy to Japanese cedar pollens. Comparison of nasal PD in the pollen season revealed a lower PD in the allergic patients than in the normal control subjects. We challenged the patients with allergen in the non-pollen season and measured the time course change of nasal PD and rate of inhibition of PD by amiloride and indomethacin. Nasal PD reached the lowest value 15 min after nasal allergen challenge. Percent inhibition of PD by amiloride was greater without the allergen challenge than it was in those patients after allergen challenge (51.7% versus 29.4%, p less than 0.01). Indomethacin did not change PD without allergen challenge, whereas it depressed nasal PD by 25.1% after allergen challenge. These results suggest that decreased sodium absorption and increased chloride secretion occur in local allergic reactions. Both changes may contribute to the increase in fluid transport towards the lumen, and this may lead to abnormalities of nasal secretion during acute allergic reactions.