The fall of haematocrit after stepwise bleeding in rabbits and young kittens was not accompanied by an equivalent fall of plasma nitrogen. The relative failure of plasma nitrogen to decrease after haemorrhage was not attributable to the entry of diffusible nitrogen into the circulation.2. Plasma protein recovery by a combination of stepwise bleeding and viviperfusion was such as to yield values for the whole body/great vessel haematocrit ratio (F(cells)) in the normal range in adult cats but significantly below the normal range in young kittens and rabbits. If viviperfusion was not preceded by stepwise bleeding F(cells) in the normal range was obtained in young kittens and rabbits.3. The results of electrophoretic examination of pre- and post-haemorrhagic plasma were consistent with the entry of fresh protein into the circulation after haemorrhage occurring preferentially in the albumin fraction, but increase of albumin/globulin ratio was small in adult cats.4. It was concluded that the fall of haematocrit after haemorrhage in young kittens and rabbits was mainly due to the entry of protein rich tissue fluid into the circulation. In adult cats, however, the haematocrit did not always fall and any such fall may have been partly due to a shift of plasma from the periphery to the great vessels.5. The bearing of these observations on the validity of calculation of blood volume from red cell mass and haematocrit during stepwise haemorrhage was considered.
