Plasma butyrylcholinesterase (BuChE) and red cell acetylcholinesterase (AChE) activity was measured in patients with endstage kidney disease undergoing hemodialysis or renal transplantation. All patients had significantly lower values of BuChE activity than normals. A single hemodialysis did not influence the level of BuChE activity but caused a moderate, though statistically significant increase of red cell AChE activity. Chronic hemodialysis significantly increased BuChE but did not affect AChE activity. After successful renal transplantation a sharp fall of BuChE activity occurred, a completely unexpected finding. This decrease coincided with the administration of large doses of glucocorticoids. Once the dose of these drugs was decreased the activity of plasma BuChE returned toward preoperative values. No similar changes in red cell AChE were seen. The possible effect of the steroid drugs on protein synthesis in the liver is discussed.