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The amplitude, frequency and rate of rise of intra-uterine pressure cycles in rats (postpartum, ovariectomized) were unaffected by treatment with progesterone. Amplitude was also unaffected by a combination of treatments with progesterone and oestradiol-17β, which was adequate to ensure the survival of 84% of foetuses in ovariectomized pregnant rats. The failure of progesterone to influence myometrial activity could not be attributed to a lack of 'true' progesterone receptors since these were present in the myometria of the test animals in concentrations exceeding those of oestrous animals. Evidence was obtained which suggested that a high-affinity binding protein, different from the 'true' receptor may predominate in the myometrium of the pregnant rat. Oestradiol-17β in single or repeated doses of from 0·25 to 5 µg, however, was found to reduce the frequency of pressure cycles but to increase significantly their rate of rise of pressure. There was a latency of 6–8 h in these effects of oestradiol. The possibility that inhibition of the myometrium by oestrogen may play a part in the preparation for parturition is discussed.