The effects of oxytocin and prostaglandin E2 (PGE2) infusions on regional blood flow (measured by radioactive microspheres) and myometrial contractility were studied in 18 pregnant rhesus monkeys near term. We observed no significant differences between PGE2 and oxytocin in cardiac output, hemodynamics, or uterine activity. Their effects on the regional distribution of systemic blood flow were similar although the gastrointestinal tract received an increased percentage of cardiac output after PGE2. A relative placental ischemia, together with a myometrial hyperemia, was observed during labor. Uterine contraction produced a large reduction (average 73 per cent) in placental blood flow whereas myometrial blood flow was maintained or sometimes increased. A significant negative correlation was observed between intra-amniotic pressure and placental blood flow. During uterine relaxation, placental flow partially recovered whereas myometrial flow nearly doubled the prelabor values. We conclude that (1) in the dose ranges studied, oxytocin and PGE2 produce similar effects on myometrial contractility and uteroplacental blood flow, and (2) the vascular beds of the placenta and myometrium respond differently to labor of moderate intensity.