The administration of phenylhydrazine to rats brought about a marked increase in the dUTPase activity in the cytosol fractions of spleen and red blood cells; the activity began to increase with a two-day lag and reached the maximum at the 5th or 6th day of the phenylhydrazine treatment (13 and 5 times the control values in total activity in the spleen and red blood cells, respectively), and then the activity decreased. The activities of thymidine kinase and sigma-aminolevulinate synthase in the spleen and red blood cells also changed in parallel with that of dUTPase. The increases of these activities were suppressed completely by methotrexate, an inhibitor of DNA synthesis. The time courses of the enzyme activity changes in the red blood cells, however, were slightly behind those in the spleen. Thus, a close correlation was assumed between the dUTPase activity and the multiplication of erythroid cells in rat spleen.