Ovariectomized rats were given a single dose of synthetic oestrogen or progestogen, or both combined. On selected days after treatment one rat from each treatment group was killed and myometrial contractility in response to spasmogens was measured isometrically in isolated tissue baths. Contractility persisted at a low level after ovariectomy without steroid replacement (controls) for the 60 days of the experiment. The powerful potentiating action of oestrogen reached a maximum in about 10 days and sustained a high level thereafter. Progestogen treatment did not influence contractility to a significant degree when compared with saline, except that the contractions were sometimes of higher frequency and more irregular in size. There was a less powerful potentiating action on contractility with combined oestrogen and progestogen treatment than with oestrogen alone. It reached a maximum in 4 days and declined more rapidly than with the oestrogen treated preparation but was still vigorous for up to 30 days.