1 Prostacyclin (PGI(2)) production by chopped segments of rat pregnant uterus was low compared with synthesis by separated myometrial tissue. Incubation of separated myometrium with decidua (2:1 by weight) led to an inhibition of myometrial PGI(2) output.2 Boiling decidual tissue abolished the inhibitory influence on myometrial PGI(2) output. Preincubation of decidua with 5,8,11,14-eicosatetraynoic acid (ETA) (30 mug/ml) also suppressed decidual inhibitory activity but indomethacin (30 mug/ml) was ineffective.3 Incubation of decidual and myometrial tissue with arachidonic acid (AA) 10 mug/ml did not increase the inhibition of myometrial PGI(2) synthesis, even if the decidua were pre-incubated with indomethacin.4 Myometrial PGI(2) production was reduced if the chopped tissue was pre-incubated with soya bean lipoxidase for 10 min at 4 degrees C. This reduction was reversed if the lipoxidase was incubated with ETA (30 mug/ml) for 30 min at 37 degrees C before addition to the myometrial tissue.5 Perfusion of the uterus to remove blood elements removed the inhibitory action that the decidua exerted upon myometrial PGI(2) production. PGI(2) synthesis by separated decidual and whole uterine tissue was markedly elevated.6 The addition of rat blood platelets (0.75 x 10(9)/ml) to incubations of perfused decidual tissue reduced PGI(2) output and restored the inhibitory action that the decidua exerted on myometrial PGI(2) synthesis.7 It is concluded that a lipoxygenase enzyme contained in blood platelets trapped within the decidual vasculature produces a hydroperoxy acid which inhibits decidual PGI(2) production or myometrial PGI(2) synthesis when the tissues are incubated together. It is suggested that perfusion is a pre-requisite before study of PGI(2) synthesis in highly vascularised tissues.8 The pathophysiological importance of such platelet lipoxygenase products is discussed.