By means of cytochemistry lactate dehydrogenase (LDH), alkaline and acid phosphatase (AP, SP) were identified in megakaryocytes of the rat. Even the smallest dosage of cyclophosphamide caused a decrease of LDH and AP activity, while the activity of SP increased. Simultaneously, the number of thrombocytes declined. Higher dosage corresponded to loss of LDH activity; in contrast, AP and SP remained unaltered. The reduction of LDH activity is suggested to be due to inhibition of enzyme synthesis caused by RNA inhibition. Increase of SP activity is interpreted as a morphological sign of enhanced metabolism or catabolism in the megakaryocytes