There is increasing evidence that the antiprogesterone mifepristone (RU-486) can dilate the cervix of pregnant women. The uterine and cervical smooth muscle contractile response to adrenergic agonists is regulated by the steroidal environment. This study was undertaken to assess the effects of treatment with RU-486 on the concentrations of alpha- and beta-adrenoceptors in cervical crude membranes from pregnant women using a radioligand binding assay. A special needle biopsy technique was used for human cervical specimens. The probable relative oestrogen dominance due to the antiprogesterone treatment selectively decreased the alpha-2 adrenoceptor in human cervix at an early stage of gestation. This finding was similar to that reported earlier in pregnant rabbits. The existence of a functionally distinct alpha-2 adrenergic receptor subtype will have important implications for our understanding of the contractile activity of the cervix.