The behavioral effects of MK-801 were compared in morphine-dependent and non-dependent mice. The dose of MK-801 selected for these studies was previously demonstrated to attenuate some of the morphine withdrawal signs. Subjects were repeatedly exposed to morphine (8 days, b.i.d., 10-100 mg/kg, s.c.). Twenty-four hours after last morphine injection mice received naloxone (0.1 mg/kg, s.c.) and the observation was commenced. Animals were pretreated with either MK-801 (0.1 mg/kg, i.p.) or saline 30 min prior to testing. It was found that the behavioral effects of MK-801 (decreased sociability, and increased rate of transitions between behavioral elements, locomotion, grooming) were less pronounced in morphine-dependent compared to non-dependent subjects. However, the intensified almost stereotypic eating possibly reflected increased psychotomimetic potency of MK-801 in morphine-withdrawn animals.