Measurements of physiological deadspace/tidal volume ratio (VD/VT per cent) and alveolar-arterial oxygen tension difference (A-aPo2) were made during mechanical ventilation in patients undergoing open-heart surgery. The inspiratory: expiratory time ratios studied were 0.5:2.5 and 1.5:1.5 before perfusion and 0.5:2.5, 1:2 and 1.5:1.5 after perfusion. Pre-perfusion there were no significant differences in VD/VT or A-aPo2 between the two patterns of ventilation. Post-perfusion the A-aPo2 increased. Although there were no significant differences in A-aPo2 between any of the three ratios examined after perfusion, the VD/VT per cent was significantly less when the inspiratory time was 1.5 seconds than when it was 0.5 or 1.0 second.