In a previous paper the readiness with which the blood of a new-born infant hemolyzes as compared with that of older persons was demonstrated and attributed to the fact that the fetus, by the admixture of arterial and venous blood in antenatal life, received an insufficient O supply, which was compensated by polycythemia; that after birth, arterial and venous blood being no longer mixed, the necessity for polycythemia ceased; and that this led to destruction of the excess of red cells and thus to icterus neonatorum. To test the correctness of this theory of the connection between reduced O supply and polycythemia, and between restoration of O supply and jaundice, 13 young guinea pigs were kept 2-4 wks. under reduced atmospheric pressure, and 5 others were kept as controls. A polycythemia developed rapidly in the animals under reduced pressure, reaching its maximum in about 10 days. The average increase of the red blood cells as well as the hemoglobin was about 30% above the original. Return to normal took place in about 5 to 8 days after the pressure was increased to normal. There was with the polycythemia a marked increase in reticulated red blood cells, 5 to 6 per high power field. When the pressure was increased to normal the reticulated cells disappeared in about 1 wk. There was an increase in the icteric index and an appearance of positive indirect van den Bergh reactions after the animals were removed from the low atmospheric pressure chamber. The icteric index ranged from 7 to 9 (normal 1 to 3) and the van den Bergh reaction from 1 to 2 units (normal O). As in new-born infants, the animals released from the environment of lowered atmospheric pressure very soon showed an increasing icteric index and a positive indirect van den Bergh reaction, together with a reduction of the polycythemia and the disappearance of the reticulated cells. Thus an icterus, similar to icterus neonatorum, was produced in animals. The period of re-adjustment was of somewhat shorter dura- tion than in the human fetus. No nucleated red blood cells were found in these animals and the fragility of their red cells showed no changes. It would therefore appear reasonable to conclude that icterus neonatorum is a hemolytic icterus, the result of a postnatal re-adjustment from a condition of O unsaturation to a normal O saturation.