The spleen of the exhypoxic polycythemic mouse was employed to study the effect of erythropoietin on the synthesis of chromosomal proteins. At 1, 3, 18, 36, 45, and 72 hr after injection of erythropoietin, spleens were removed, minced, and incubated with 3H-arginine for 1 hr. Chromatin was isolated from the labeled splenic tissue and separated into histone and nonhistone protein (NHP) fractions. An increase in the incorporation of 3H-arginine into NHP occurred within 3 hr and into histones by 18 hr. Incorporation of 3H-arginine into both histones and NHP was maximal by 45 hr and had declined by 72 hr. Total histone specific activity increased fivefold while NHP specific activity increased twofold. Histones and NHP were fractionated on polyacrylamide gels and a double isotope labeling proceudre was used to study the synthesis of the individual histone proteins and NHP. Following administration of erythropoietin, there was a coordinate increase in the specific activity of all five major histone proteins and most of the NHP. The earliest change in specific activity of both histones and NHP occurred prior to the appearance of morphologically identifiable erythroblasts in the spleen.