Baseline airway geometry is thought to be an important determinant of the airway response to challenge; this geometry is altered by changing posture. The effect of changes in posture on airway calibre, midtidal lung volume, and the airway response to inhaled histamine was studied in eight healthy subjects (four female; mean (SD) age 29.8 (5.1) years, FEV1 3.54 (0.65) 1). Each subject was studied in both sitting and supine postures on two days; airway calibre was assessed by measuring total respiratory resistance (Rrs) at 6 Hz with a forced oscillation technique applied over 16 seconds of tidal breathing. Appropriate doses of histamine were selected by preliminary experiments and were always inhaled with the subject in the supine posture. Midtidal lung volume was larger in the sitting (2.9 (0.8) 1) than in the supine posture (2.4 (0.5) 1). Baseline Rrs was lower in the sitting than in the supine posture (2.03 (0.44) and 3.12 (0.76) cm H2O.1(-1).s*). The mean absolute increase in Rrs after the same dose of histamine was 1.22 cm H2O.1(-1).s in the sitting position (65.8% increase over baseline) and 1.39 cm H2O.1(-1).s (48.8% increase over baseline) in the supine position. The geometric mean provocation concentration of histamine causing a given percentage increase in Rrs was similar in the sitting (8.26 mg/ml) and supine (8.65 mg/ml) positions. Thus there was no significant increase in responsiveness after the reduction of airway dimensions and extra-airway distending forces that occurs in the supine posture.