The effect of postoperative extradurally administered morphine on lower urinary tract function was studied in female patients undergoing uterine surgery. Urodynamic measurements were made on the day before and on the day after the operation, using a DISA 2-channel carbon dioxide (CO2) cystomictrograph. In ten patients without postoperative urinary retention no changes in cystometry were found during morphine administration, while two patients who developed acute urinary retention had a marked increase in bladder capacity and of detrusor pressure. In contrast, the urethral pressure profile was unchanged in both groups of patients. Intravenously administered naloxone tended to normalize the bladder capacity in the patients with urinary retention. These findings seem to indicate a marked effect in some patients of extradurally administered morphine and the acute urinary retention, following morphine administration, may be treated with naloxone.