The incidence of neonatal morbidity and mortality in rats exposed to opiates in utero is generally high. To determine the extent to which neonatal opioid intoxication and/or withdrawal contribute to this effect, addicted pups from dams treated chronically with the long-acting opioid levo-alpha-acetylmethadol (LAAM) and appropriate controls were injected within 12 h of birth with saline, an opioid agonist (LAAM and metabolites) or an antagonist (naloxone). The incidence of neonatal mortality for pups born to dams maintained on a high dose of LAAM was 52%. A single injection of agonist on the first day of life reduced mortality in this group to 29% while a single injection of the antagonist increased mortality to 88%. In contrast, administration of the agonist to control pups and pups born to dams maintained on lower doses of LAAM resulted in increased mortality. Naloxone was apparently innocuous in non-dependent neonates. These data show that, despite LAAM's long duration of action in the mature rat, newborn rats experience withdrawal soon after drug exposure is terminated. These data also indicate that continued opioid exposure is a highly effective means of treating/preventing severe spontaneous withdrawal in the newborn.