Aim: Clonidine is an α2 adrenoceptor agonist that is frequently used to reduce withdrawal symptoms during opioid detoxification in humans. The long-term effects of clonidine on withdrawal symptoms and its effects on subsequent drug exposure have not been thoroughly documented. The aim of the study was to determine if clonidine administered during morphine withdrawal in rhesus mon-keys produces long-lasting effects on withdrawal symptoms and alters the effects of subsequently taken drugs of abuse. Methods: Adult male rhesus monkeys were treated with increasing doses of morphine for 90 d to induce opiate (narcotic)dependence. The immediate and long-lasting effects of 1 week’s administration of clonidine were measured via the recording of morphine withdrawal signs and the subsequent effects of challenge injections of morphine or cocaine. Results:Monkeys chronically treated with morphine displayed withdrawal signs that lasted 2 weeks after cessation of morphine administration and displayed sensitized re-sponses to subsequent morphine and cocaine injections. Clonidine significantly reduced certain morphine withdrawal signs and overall withdrawal score, but these effects did not persist upon cessation of clonidine treatment. Sensitization to the effects of morphine and cocaine were significantly reduced in monkeys previ-ously treated with clonidine. Conclusion: Our results suggest that in addition to its short-term alleviating effect on morphine withdrawal signs, clonidine may re-duce subsequent effects of drugs of abuse after prolonged abstinence.