Effects of morphine on spontaneous activity of 18-day rat fetus.

The effects of morphine on spontaneous activity of 18-day fetal rats have been observed in utero. Morphine injected subcutaneously in the dam caused a decrease in fetal spontaneous activity which was dose-dependent. The maximum depression of activity occurred after 20 mg/kg was administered to the dam. Activity returned to control levels following naloxone administration to the dam. Fetuses which had been chronically exposed to morphine from days 7 through 17 and observed on day 18 of gestation became hyperactive following administration of naloxone to the dam. This is thought to represent fetal withdrawal.