Nasal patency is usually assessed in the laboratory by measuring nasal airflow conductance (Gnaw); peak inspiratory and/or expiratory flow measurements via the nose (PIFna, PEFna) have been proposed as simple alternatives suitable for home monitoring of rhinitis. We have compared the scale of changes in PIFna and PEFna (measured with a pneumotachograph) with changes in Gnaw (measured by the forced-oscillation technique) when nasal patency was increased by a topical alpha-adrenergic agonist, xylometazoline (five control subjects, seven with seasonal rhinitis, studied when asymptomatic) or decreased by topical histamine (eight control subjects). In further experiments, we altered intrapulmonary airway calibre by having subjects inhale histamine or salbutamol aerosols and examined effects on the configuration of nasal flow-volume curves (six subjects with rhinitis and mild asthma). After topical xylometazoline, there was a mean 283% increase in Gnaw, 80% increase in PEFna, and 63% increase in PIFna. After topical histamine, there was a mean 72% decrease in Gnaw, 38% decrease in PEFna, and 39% decrease in PIFna. Inducing intrapulmonary airway obstruction sometimes obscured changes in nasal patency by removing the effects of added nasal resistance on expiration and preventing development of flow limitation in the nose on inspiration. Thus, after topical drug treatment to the nose, changes in Gnaw were considerably larger than in PEFna or PIFna, which were proportionately similar. Because PIFna is usually restricted by nasal flow limitation, it is probably superior to PEFna for assessing nasal patency. When effort is submaximal, intrapulmonary dynamic resistance is increased, or nasal dynamic resistance is low, PEFna and PIFna can give a misleading impression of nasal patency. These errors can be avoided by comparisons with mouth PEF and/or PIF, suggesting that nasal and mouth peak flow should both be measured during home monitoring.