Continuous distributions of ventilation-perfusion (VA/Q) ratios were measured in ten subjects with moderately severe symptomatic asthma. Six of the subjects had only minimal VA/Q inequality (mean log SD of bloodflow 0.5) despite having airways obstruction similar to that in the four subjects with marked VA/Q inequality (mean log SD of bloodflow 1.0). The six patients with minimal VA/Q inequality developed marked widening of their VA/Q distributions while breathing 100 percent oxygen (mean log SD bloodflow 1.1), and four of these patients maintained more modest widening after receiving an intravenous antihistamine, clemastine (mean log SD bloodflow 0.75). The four subjects with a wide control VA/Q distribution showed smaller changes while breathing pure oxygen and no change after receiving clemastine. FEV1 improved with clemastine treatment in the first four patients only. The results suggest that the majority of patients with moderately severe asthma have compensatory pulmonary vasoconstriction, causing better VA/Q matching which is responsive to hypoxia and, possibly, histamine. The data demonstrate a relationship between active compensatory vasoconstriction and airway sensitivity to antihistamine.