Plasma choline levels were measured in patients who received a kidney transplant, in donors who underwent nephrectomy and in nonrenal surgical patients. Choline was measured using a choline kinase assay. Choline levels in patients receiving a kidney fell from 29.8 ± 1.86 µM before transplantation to 15.7 ± 2.32 1 day later; this normal level was maintained for at least 7 months and in a single case for 2 years. Kidney donors and nonrenal surgery patients showed a significant decrease in plasma choline on the day following surgery but choline levels returned to normal by 3 days after surgery. Thus a transplanted functional kidney reduced the high plasma choline levels, associated with uremia, to normal and maintained these normal levels throughout the period of observati