A polarographic technique for acute and chronic recordings of fetal and maternal oxygen availability (aO2) was developed. Fetal brain oxygen availability equaled maternal and neonatal oxygen availability when the uterus was quiescent. During uterine contractions or when pressure was applied to the fetal head, O2 availability was greatly diminished. During labor the fetal O2 diminished considerably but recovered rapidly after delivery. Apresoline, Levophed, adrenaline, Pitressin and Pitocin produced different effects on fetal and maternal brain O2 availability. These differences can be accounted for by the known hemodynamic and/or uterine actions of these drugs.