The present study was performed on rats, mainly to examine the so-called priming effect on megakaryocytopoiesis. One group of animals received 2 or 4 mg thio-TEPA or 200 mg cytosine arabinoside/kg body weight (the pretreatment) 2.5 days or 8 days prior to a dose of 10 mg thio-TEPA/kg body weight (the challenge dose). Another group received a pretreatment dose of 1 mg melphalan/kg body weight 2.5 days prior to a challenge dose of 3 mg melphalan/kg body weight. The number of bone marrow megakaryocytes, blood platelet production, mean platelet volume, blood platelet counts, leucocyte and granulocyte counts were examined on days 2, 4, 7, 10, 13, 16 and 20 after the challenge dose. The gut mucosa (number of mucosal crypts in terminal jejunum) and survival were studied in animals receiving pretreatment 2.5 days prior to a challenge dose of about LD100 for thio-TEPA and melphalan. No systematic differences were observed whether the animals received pretreatment prior to the challenge dose or not. Thus, no priming effect was observed.