Tamoxifen and the role of oestrogen in the timing of parturition in the rat.

After tamoxifen treatment 'spontaneous' pressure cycles were resumed in conscious rats in which myometrial activity had been inhibited with oestradiol benzoate, demonstrating an anti-oestrogenic effect of this compound. Tamoxifen did not shorten the length of pseudopregnancy to less than 11 days when administered to rats 4 days after cervical stimulation. The effect of oestradiol benzoate (OB) treatment of pregnant rats depended on the day of administration. On Day 18 OB injection (5 micrograms) advanced the mean time of parturition by 5-6 h (P less than 0.001) whereas injection on Days 19 or 20 delayed delivery by more than 6.5 and 8.5 h respectively (P less than 0.05). It is suggested that the ability of tamoxifen given on Days 20 or 21 to advance the time of parturition in rats is due to its anti-oestrogenic activity.