In 59 full term new-born infants an increase in the mean value for (blood, (B] B-haemoglobin, (plasma, (P] P-potassium and (plasma, (P] P-bilirubins was seen during the first three days of life. The erythrocyte volume fraction, P-sodium and P-albumin varied in parallel with the B-haemoglobin concentration. The variation in these blood constituents indicated that an initial destruction of erythrocytes occurred and that a haemoconcentration took place as well. This was further substantiated by the observation of a weight loss in the infants of about 6.4% of the estimated total body water content during the first three days after delivery. An increase in P-sodium of 5% in the same period indicated that the weight loss consisted mainly of water. These results show that new-born infants are prone to be hypovolaemic during the first three days of life.