The DNA content of rat bone marrow megakaryocytes (MK) was studied by Feulgen photometry following splenectomy and sham operation, respectively. The DNA measurements were preceded by acetylcholinesterase staining for identification of the 2N-8N MK. The number of 2N-8N MK decreased to minimum values, while the number of 16N- 64N MK increased to maximum values about 4 days following both splenectomy and sham operation. However, the changes were somewhat more pronounced following splenectomy than sham operation. The total MK number did not change significantly. Platelet production, measured by 35S incorporation into platelets, increased during the first 2 days and remained high for 6–7 days, increasing the platelet counts. All values were about normal 30 days after surgery, except for a minor thrombocytosis following splenectomy. The early, highly significant thrombocytosis, following both splenectomy and general surgery, is caused by increased production of platelets due to the surgical trauma. This is caused by a direct action on bone marrow MK by transforming 2N- 8N MK into higher ploidy classes. Lack of splenic platelet pooling may influence the grade and duration of the early thrombocytosis after splenectomy. The late, long-lasting, minor thrombocytosis, which occurs after splenectomy but not after sham operation, can be explained by the removal of the splenic platelet pool.