Pruritus (itch sensation) is a significant clinical problem. The aim of this study was to elucidate the roles of opioid receptor types and the site of action in opioid-induced itch in monkeys. Observers who were blinded to the conditions counted scratching after administration of various drugs. Intravenous (i.v.) administration of mu opioid receptor (MOR) agonists (fentanyl, alfentanil, remifentanil, and morphine) evoked scratching in a dose- and time-dependent manner. However, the kappa opioid agonist U-50488H [trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide] and delta opioid agonist SNC80 [(+)-4-[(alphaR)-alpha-[2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-methoxybenzyl]-N,N-diethylbenzamide] did not increase scratching. Intrathecal (i.t.) administration of peptidic MOR agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 0.00032-0.01 mg) evoked scratching, but i.v. DAMGO (0.01-1 mg/kg) did not increase scratching. A similar difference between i.t. and i.v. effectiveness was seen with morphine. Antagonist studies revealed that i.v. administration of an opioid receptor antagonist (naltrexone, 0.0032-0.1 mg/kg) dose dependently attenuated scratching induced by i.v. fentanyl (0.018 mg/kg) or morphine (1 mg/kg). However, a peripherally selective opioid antagonist (quaternary naltrexone, 0.0032-0.32 mg/kg) did not block i.v. fentanyl- or morphine-induced scratching. Moreover, a histamine antagonist (diphenhydramine, 0.1-10 mg/kg), failed to attenuate scratching induced by i.t. morphine (0.032 mg) or i.v. morphine (1 mg/kg). Pretreatment with a selective MOR antagonist (clocinnamox, 0.1 mg/kg), but not kappa or delta opioid antagonists (nor-binaltorphimine or naltrindole), blocked i.t. morphine-induced scratching. Together, these data suggest that MOR, not other opioid receptor types or histamine, mediates scratching evoked by opioid analgesics. More important, this study provides in vivo pharmacological evidence that activation of central MOR plays an important role in opioid-induced itch in primates.