The present study describes the effects of the inhibition of adenosine deaminase and of adenosine kinase on reticulocytes and erythrocytes of rabbits. Both in erythrocytes and in reticulocytes the degradation of adenine nucleotides proceeds via AMP----IMP----inosine----hypoxanthine. Under approximately physiological conditions the rate of degradation amounts in erythrocytes to 23 mumoles/l cells.h and in reticulocytes to 331 mumoles/l cells.h, respectively. In erythrocytes the formation of hypoxanthine corresponds closely to the degradation of adenine nucleotides in reticulocytes; the formation of hypoxanthine seems to exceed the degradation presumably mainly due to RNA degradation. Parallel to the primary deamination of AMP there is a primary dephosphorylation to adenosine of about 60 mumoles/l cells.h in erythrocytes and about 300 mumoles/l cells.h in reticulocytes. This pathway does not provide, however, any measurable contribution to the formation of hypoxanthine, because the adenosine formed is rephosphorylated via adenosine kinase almost completely.