Antagonism of morphine by naloxone in pregnant ewes and fetal lambs.

We wanted to determine if naloxone antagonizes the direct effects of morphine transferred from the mother to her fetus and if this antagonism resembles fetal narcotic withdrawal. After continuous infusions of morphine into pregnant ewes, injections of naloxone into their fetuses produced increases in fetal arterial blood presure; decreases in fetal heart rate, arterial blood pH, and PO2; and defecation of meconium. There were no maternal responses. Injections of naloxone into the mothers produced the same fetal responses and narcotic withdrawal responses in the mothers. Naloxone, therefore, antagonized the direct effects of morphine in the fetus, and this antagonism produced responses resembling narcotic withdrawal.