The injection of 10 .mu.g angiotensin [A]II to 5 near-term sheep caused significant increases in the renal, uterine and placental vascular resistances. After pretreatment with 10 mg/kg indomethacin the injection of 10 .mu.g AII caused a significantly greater change in the vascular resistance of the kidneys and uterus. The response of the placental vasculature to AII was not affected by the pretreatment. Endogenous prostaglandins [PG] may play a role in modulating the response of the renal and uterine vascular beds to AII but this study provides on evidence to support the hypothesis that PG modulate the placental response to AII.