Daily variations of plethysmographic measurements of airway resistance were studied in normal subjects and in patients with obstructive respiratory disease. Airway resistance seems to be highest at 7 a.m., immediately after awakening, and decreases in the first hour by about 20% in normal subjects and more than 60% in pathological subjects. No significant variations of RAW were noted during the day in normal subjects, while bronchitics and asthmatics showed an increase of airway resistance in the afternoon. In these patients the difference between airway resistances measured in the morning at about 10 a.m. and in the afternoon at about 4 p.m. is statistically significant (P < 0.05).