ExtractAdditional lipids have been tested to determine whether they share with soya bean phospholipids (SBP) and dilinoleyl lecithin (DLL) the ability to sensitize the pregnant rabbit uterus to oxytocin, which permits the induction of premature labor at a time otherwise not possible. Comparable activity was given by none of the compounds tested intravenously but was given by methyl linoleate and methyl arachidonate administered by the intrauterine route, both at a dose of 1 g; abortion rates for these compounds were 69% ± 4.3% and 79% ± 7.8%, respectively. These values do not differ significantly from those for intravenous SBP and DLL.SpeculationAll highly active compounds thus far found have either linoleate or arachidonate as a part of their molecule. These essential fatty acids are precursors of prostaglandin E or F2α, both of which can stimulate myometrial contractions in the pregnant rabbit uterus and cause luteolysis. We postulate that active lipids act by conversion to prostaglandins, which in turn prepare the uterus for delivery. Both the time interval required to achieve this effect and the ability of progesterone to block it suggest that the prostaglandins act by luteolysis rather than by stimulation of myometrial contractions.