The authors were investigating the metabolism of prostaglandin F<sub>2α</sub> in human early placenta, in in vitro incubation experiments. They have identified both major metabolites, 15-keto-PGF<sub>2α</sub> and 15-keto-13, 14 dehydro PGF<sub>2α</sub>. They have examined the change in the percentage of these, taken as a function of time. They have established that the main site of inactivating the endogenous PGF<sub>2α</sub> is the early human placenta. They suppose that the considerable Pg-inactiviting effect of the placenta has a part which acts as a protecting mechanism in preventing the uterotonic influence of the endogenous prostaglandins.