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Ovariectomy of rats on day 20 of pregnancy impaired parturition and lactation. All rats showed prolonged delivery with many foetuses born dead and the mothers were unable to rear the young. Treatment with oestradiol benzoate (0·5 or 1 µg) in a single dose permitted normal delivery and lactation when the hormone was injected immediately after ovariectomy. Oestrogen administered the day after ovariectomy was not effective. The administration of prolactin and corticotrophin to ovariectomized pregnant rats did not prevent abnormal parturition but lactation improved in 50% of the mothers. A group of animals ovariectomized on day 20 was treated with oxytocin every 2 h on day 22 starting at 08.00 h. This treatment did not facilitate parturition and lactation. Maternal behaviour, which was always present in the other groups, was also impaired by this treatment. When ovariectomy was performed on the evening of day 21 of pregnancy, a partial impairment of parturition was observed but lactation was normal in seven out of eight rats. The results indicate that oestrogen is essential near term for normal parturition and lactation. The absence of the steroid may make the uteri less sensitive to oxytocic substances and also affect milk ejection.