Chronic treatment with an opioid antagonist, such as naltrexone, increases opioid receptor density and opioid agonist potency. Since stimulants such as d-amphetamine can increase opioid potency and opioid abusers may administer stimulants during naltrexone treatment, the effect of chronic d-amphetamine on naltrexone-induced opioid receptor upregulation and supersensitivity was examined in mice. Mice were implanted s.c. with a 15 mg naltrexone or placebo pellet for 8 days. Mice were injected daily with saline or d-amphetamine (7.5 or 5.0 mg/kg per day s.c.) for 7 days beginning 24 h following implantation. Naltrexone and placebo pellets were removed on the 8th day, and 24 h later mice were tested for morphine analgesia (tail-flick) or whole brain was removed and opioid receptor binding studies were conducted. Chronic naltrexone significantly enhanced the analgesic potency of morphine in saline-treated mice. However, naltrexone treatment did not increase morphine potency in mice treated with d-amphetamine. In binding studies, naltrexone increased [3H][D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) Bmax (+60-70%) without altering KD in both saline- and d-amphetamine-treated mice. Results from studies with 2 nM [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) were similar. These studies indicate that daily d-amphetamine can limit naltrexone-induced supersensitivity but not receptor upregulation. Thus, upregulation can be dissociated from functional supersensitivity.