The authors evaluated the effects of an anticholinesterase, tetrahydroaminoacridine (THA), in mice on morphine-induced analgesia (by the hot-plate test) and on naloxone-precipitated withdrawal in morphine-tolerant mice. THA (3 mg/kg) had no analgesic effects of its own and, when given concurrently with morphine, THA did not affect the degree of analgesia, but it did prolong analgesia. Naloxone did not elicit withdrawal signs in THA-treated mice, but the signs in mice chronically treated with morphine and THA were greater than in those given morphine alone. These results only partially support other reports dealing with cholinergic modulation of opiate effects. Differences may be due to the fact that THA has some effects other than cholinesterase inhibition.