Anatomical and physiological deadspace increased significantly following intravenous administration of 0.6 mg atropine in healthy volunteers. Small increases in respiratory minute volume maintained constant values for alveolar ventilation and arterial Pco2. Atropine did not alter the arterial Po2 or the magnitude of the alveolar-arterial Pco2 gradients during breathing of air, 100 per cent oxygen or 11 per cent oxygen. It is concluded that atropine increases the degree of regional relative overventilation, but not the degree of regional relative overperfusion.