Intra-amniotic injection of pregnant rat mothers with pargyline HCl (Eutonyl), a MAO blocker, produces fetal death through activation of endogenous serotonin. In the present paper 5 progesterone derivatives were tested for their capacity to prevent serotonin abortion. While progesterone, dydrogesterone, medroxy-progesterone acetate and allylestrenol were not effective in preventing serotonin abortion, treatment with the retroprogesterone trengestone (RO 4-8347) prevented abortion and premature delivery in the 2nd and 3rd trimesters, though it could not prevent serotonin abortion in rats in the 1st trimester. The fetus-saving effect of trengestone appears to be due to the fact that it increases formation of serotonin-metabolizing enzymes, including MAO in the endometrium, whereas estrogens decrease it.