Acetaminophen is frequently administered orally to children for its analgesic properties, although its potency has never been clearly evaluated in this population. In certain situations (patients vomiting or unconscious), acetaminophen has to be given rectally. However, the rectal absorption of suppositories is frequently erratic. We undertook this study first, to measure the absorption of an aqueous solution of acetaminophen administered rectally. Secondly, we evaluated acetaminophen's postoperative analgesic effects in children aged 1 to 8 years old undergoing adenoidectomy and/or tonsillectomy and compared its efficacy to meperidine. Twenty children received 20 mg.kg-1 of acetaminophen at the time of induction of anaesthesia while 20 others received 1 mg.kg-1 of meperidine intramuscularly. Thirty-two patients required meperidine in the Recovery Room. There was no statistical difference between the patients who received acetaminophen (18), and those who received meperidine (14). The absorption of acetaminophen was incomplete (peak serum concentration: 70.8 mumol.L-1) and delayed. We conclude that the rectal administration of acetaminophen at the induction of anesthesia results in incomplete and delayed absorption and does not prevent the occurrence of immediate postoperative pain in children undergoing adeno-tonsillectomy.