The3H-thymidine labeling index (3H-TdR-index), the concentration/0.1 mm2 tissue area and the diameters of splenic megakaryocytes were determined in rats after 2/3 partial hepatectomy, sham operation or intravenous antigen injection (sheep erythrocytes). During the first 36 hours after partial hepatectomy a slight, though not significant decrease of the3H-TdR-index was noticed. The maximal values were obtained at the third postoperative day. The changes in the megakaryocyte numbers showed large variations and only at the eighth day a significant increase was visible. No makromegakaryocytes were observed and cell diameters did not increase. However, at days 4, 6 and seven a significant decrease of megakaryocyte sizes developed. After sham operation an immediate rise of the labeling index of splenic megakaryocytes with a maximum 36 hours postoperatively occured, whereas cell numbers and diameters changed only slightly. The injection of antigens had no measurable effect on megakaryocytopoiesis. As after other kinds of tissue damages megakaryocyte maturation time was reduced in sham operated and partial hepatectomized animals thus obviously compensating an increased peripheral platelet demand (wound repair, blood loss). At what level and to what extent the megakaryocyte renewal system in the spleen was affected by the partial hepatectomy as well as the nature of the observed initial delay in radioactive labeling cannot be concluded from this investigation with certainty. But it may well be correlated with a temporally reduction of the platelet sequestration capacity of the liver after partial hepatectomy. The rate of peripheral destruction of platelets may influence platelet production and megakaryocytopoiesis independantly of the platelet count.