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The antioestrogen tamoxifen, widely used in the treatment of breast cancer, and oestradiol rapidly inhibited contraction of isolated rat myometrium induced by many kinds of spasmogens eg. Oxytocin, acetylcholine, potassium and cold contracture showed that the inhibition probably involved a common pathway. The inhibition was too rapid in onset (~ 3 min) to be via the classical oestrogen receptor pathway followed by changes in RNA transcription and protein synthesis. Moreover, the finding that oestradiol which has higher affinity to its own receptor than tamoxifen could not prevent or even reduce inhibition effect of tamoxifen but the interaction was additive supported the above conclusion. Rapid and complete recovery from oestradiol inhibition and generally good recovery, after a certain time, from tamoxifen inhibition indicated that the effect was not due to cytotoxicity.