The cholinergic system has been proposed to participate in the development of dependence on opioids. The present study examined effects of dermal pretreatment with methyl parathion (MP), an acetylcholinesterase inhibitor, on the development of physical dependence on morphine. Opioid dependence was induced by continuous intracerebroventricular (i.c.v.) infusion of morphine (26 nmol/l/h) for 3 days in adult male Sprague-Dawley rats. Each rat received two doses of MP, 12.5 mg/kg, dermally, initially, 3 days prior to initiation of i.c.v. morphine infusion and again on the first day of infusion. Withdrawal was precipitated after 3 days of infusion by administering an opioid antagonist, naloxone (48 nmol/5 l, i.c.v.). Twelve of 23 MP-treated rats exhibited signs of acetylcholinesterase inhibitor intoxication (mild tremors) and showed reduced spontaneous locomotor activity (tested by an open field test), prior to naloxone. The brain cholinesterase activity in these 12 rats was 13% of levels in control rats. Eleven rats that did not show toxic signs, exhibited cholinesterase activities that were 20% of control (not significant versus toxic group). The group that showed signs of MP intoxication exhibited a significantly lower incidence of opioid withdrawal jumping, rearing and wet dog shakes compared with the non-toxic group. No differences between quantal withdrawal signs (ptosis, penis-licking, and vocalization) were noted between the two groups. The results suggest that toxic inhibition of acetylcholinesterase non-specifically reduces locomotor activity and may obscure certain behavioral signs of withdrawal from opioid dependence. This indicates that caution should be used in interpreting a direct involvement of acetylcholinesterase inhibition in preventing opioid dependence.