To evaluate the concentration-effect relationships of propofol during recovery after total intravenous anesthesia, 20 female patients undergoing lower abdominal surgery were studied. In 10 patients (Group B) the propofol infusion was supplemented with an epidural block with bupivacaine to evaluate the relation between the blood concentration of propofol and various pharmacodynamic end-points. The remaining 10 patients (Group A) received an alfentanil infusion intravenously instead of the epidural block to assess the dynamic interactions of alfentanil and propofol. Postoperative performances (drowsiness, amnesia, cooperation, and orientation) were evaluated by means of scoring scales. Critical flicker fusion threshold (CFF) also was used to assess the level of postoperative alertness. A propofol blood concentration of 2.5 micrograms/mL was required for satisfactory hypnosis during surgery and at 0.8 +/- 0.4 microgram/mL, the patients were considered fully awake. A concomitant alfentanil infusion reduced the propofol concentration required by 0.2-0.4 microgram/mL for the same degree of effect. Rapid recovery was seen in all patients, but in the group receiving alfentanil infusion there was a shift to the left of the concentration-effect curve in regard to drowsiness and a statistically significant prolongation of recovery by CFF-measurement which suggests a possible dynamic interaction with alfentanil. We conclude that there is a good correlation between the blood concentration of propofol and the pharmacodynamic responses during recovery.