The effect of oral treatment of rats for 12 consecutive days with the contraceptive steroids, mestranol (0·08 mg/rat/day) and lynoestrenol (2·5 mg/rat/day), separately or in combination, on the sensitivity of the isolated spontaneously contracting rat uterus to the inhibitory action of isoprenaline, adrenaline and noradrenaline was investigated. The sensitivity to isoprenaline did not change significantly after any of the hormonal treatments. The sensitivity to adrenaline was significantly reduced after treatment with mestranol and lynoestrenol separately and together. Only mestranol treatment also resulted in a significant decrease in sensitivity to noradrenaline while the decrease after lynoestrenol or mestranol+lynoestrenol was not significant. Treatment with only one or with a combination of the steroids resulted in the appearance of excitatory α-adrenoceptors which most probably contributed to the decrease in the sensitivity of the uterus to the inhibitory action of the sympathomimetic agents.