Changes in haematocrit value in rabbits subjected to pneumothorax were studied. Significant increases and decreases were noted following pneumothorax and desufflation respectively. These are attributed to modifications of lung and systemic haemodynamics, resulting in diminished reflux to the heart and a consequent increase in capillary hydrostatic pressure; the latter provides the premisses for inspissation of the blood due to displacement of water towards the interstitial space.