The number and affinity of ?-adrenoceptors on rat uterine smooth muscle membranes were determined by radioligand binding technique. The number of ?-adrenoceptors (shown by maximum binding capacity RT) of uterus in proestrus and estrus stage (126.84?54.69 fmol/mg preicin) was significantly higher than those in diestrus and metestrus stage (77.98 + 30.66 fmol/mg protein) (p<0.05). It suggests that the hormonal state of uterus may affect its receptor and thus influence the uterine contraction. There was no significant difference in affinity (shown by dissociation constant KD) among differnt stages of oestrous cycle