Effects of high hydrostatic pressure on rabbit reticulocytes themselves and their maturation phenomenon were examined. The results as follows: 1) There are no morphorologically unusual forms on compressed reticulocytes stained by the May-Giemsa's methode. 2) Remarkable difference on the rate of the reticulocyte number is not proved between the normal and the compressed blood (100-1500 atm. pressure) 3) The rate of the reticulocyte number has a tendency to be low under the pressure of about 1000-1500 atm. pressure. After all, the author has not been able to prove effects of the high hydrostatic pressure on reticulocyte maturation phenomenon is unable by these methods.