The kinetic relationship between the globin mRNA accumulation and the rate of synthesis of globin chains was studied during the terminal stages of differentiation in erythroid cells derived from the yolk sac of mouse fetuses. RNA derived from the whole cells and from different cell compartments were hybridized to DNA complementary to embryonic globin mRNA. The relative proportion of embryonic globin RNA molecules and their absolute number per cell were estimated on the 11th, 12th, and 13th days of mouse fetal development. During erythroid terminal differentiation globin mRNA became progressively predominant on polyribosomes along with the progressive specialization of cell functions. The number of embryonic globin RNA molecules per cell remained constant while yolk sac erythroid cells underwent two rounds of cell division. These data indicate that the transcription of globin genes is operative throughout the last stages of terminal differentiation and that there is no detectable storage of globin RNA sequences in these cells. The rates of accumulation of mRNA molecules and of globin synthesis both seem correlated to the length of the cell cycle.