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Simultaneous measurements of prostaglandin F, total unconjugated oestrogens and progesterone have been made in utero-ovarian venous or posterior vena caval plasma from pregnant ewes during parturition. In all cases, levels of oestrogens and prostaglandin F increased before delivery, after a decrease in the level of progesterone. In four out of six sheep, during both spontaneous and dexamethasone-induced labour, increases in the levels of oestrogens and prostaglandin F occurred in parallel. In the other two animals, levels of prostaglandin F increased before those of oestrogens. The increases in prostaglandin F consistently preceded any measurable increase in uterine activity, suggesting levels were not raised as a consequence of labour. Infusion of oestrogen to two dexamethasone-treated animals resulted in increased utero-ovarian venous prostaglandin F levels within 2–3 h. These findings support previous evidence indicating that levels of prostaglandin F may be controlled by oestrogen and progesterone. Manual examination of the cervix, with associated distension of the vagina, resulted in dramatic increases in the level of utero-ovarian venous prostaglandin F during the last 13 h of gestation, both in dexamethasone-induced labour and in labour of natural onset. Since expulsion of the foetus also results in vaginal distension, this finding raises the possibility that the very high levels of prostaglandin F observed at delivery may be caused by tactile stimulation of the vagina.