Nasal Resistance (Rn) was measured in 30 Caucasian and 13 Negro infants during the 1st year of life, using an adaptation of the posterior rhinomanometric method. Concurrent measurements of Thoracic Gas Volume (TGV) and Airway Resistance during nose breathing (Raw (n) were made using the plethysmographic technique. The percentage contribution of Rn to Raw (n) was significantly higher in the Caucasian infants (mean 49.2 +/- 7.5 (SD)%), than in the Negro infants (mean 31.1 +/- 6.8 (SD)%), which probably resulted from anatomical differences in nasal structure. In each infant, Rn was subtracted from Raw (n) in order to assess resistance, and its reciprocal, conductance (Gaw), during mouth breathing. A strong linear relationship was found to exist between Gaw (m) and TGV throughout the first year of life (r = 0.92), with no significant difference between Negro and Caucasian infants. Specific Airway Conductance during mouth breathing (SGaw (m) = Gaw (m)/TGV) was found to be considerably higher during infancy than at any other time during life, which may help to at least partially compensate for the fact that newborn infants are obligatory nose breathers.