Plasma levels of unconjugated estrone, 17β-estradiol, estriol, progesterone, 17-hydroxyprogesterone, human chorionic gonadotropin (HCG), and human chorionic somatomammotropin (HCS) were measured in patients 7–20 weeks pregnant receiving prostaglandin F2α infusions for therapeutic abortions. The results indicate that prostaglandin F2α a does not exert a luteolytic effect in terminating pregnancy of 7 weeks or more duration. Significant progesterone changes were not seen prior to abortion, and no effect was seen on 17-hydroxyprogesterone levels. There was a gradual decline in estradiol levels during prostaglandin F2α infusion, while significant falls were seen in estriol levels preceding any changes in estradiol or progesterone. There were no significant changes in HCG levels, while HCS levels declined during the infusion. The data do not rule out the possibility of a luteolytic effect for prostaglandin F2α in the first few weeks of pregnancy.