Methadone exposure in utero: effects on open-field activity in weanling rats.

In a 4 group design, pregnant rats were injected with 0, 4, or 16 mg/kg of methadone hydrochloride daily. The fourth group served as a nonhandled, nontreated control. All females in the first three groups were injected from Day 8 to gestation until term. At birth, litters were weighted, culled to eight pups each, and given to other nontreated surrogate mothers. Daily observation was made to determine infant mortality. At Day 28 of age, offspring were weaned, weighed, and separated by sex. Body weights were low at birth among the high methadone offspring and infant mortality was high. By weaning, mortality remained high, although weights were similar to controls. At weaning, high dose offspring showed depressed open-field activity when compared to low dose and control offspring. Methadone appears to cause deficits in behavioral development which can be detected at weaning.