The mechanisms that enable the myometrium to switch from a state of relative quiescence during pregnancy to a muscle that is spontaneously active, very responsive to endogenous uterotonins and exhibits a high degree of cell-cell coordination are poorly understood. It is hypothesized that this switch or 'activation' of the myometrium results from the coordinated expression of a cassette of 'contraction-associated proteins'. The molecular mechanisms that regulate the expression of one of these, namely the myometrial gap junction protein connexin-43 (Cx-43), have been analysed. Myometrial Cx-43 expression is significantly increased during labour, associated with an increase in plasma oestrogen:progesterone, and positively regulated by oestrogen in non-pregnant rats. The genomic structure of the murine Cx-43 gene and the sequence of its 5' flanking sequence are reported here. This region functions as a promoter and contains several putative cis-acting elements which may be important in the regulation of Cx-43 transcription. Among these elements are several half-palindromic sequences that may function as oestrogen response elements and several AP-1 sites that may bind the transcription factors Fos and Jun. Oestrogen treatment of cells transiently transfected with a plasmid containing the Cx-43 promoter linked to the chloramphenicol acetyl transferase (CAT) gene, increased CAT activity indicating that the murine Cx-43 gene is oestrogen responsive. In addition, treatment of rats with oestrogen significantly increased mRNA encoding c-fos and c-jun in the myometrium and this occurred before any increase in Cx-43 mRNA. These data suggest that oestrogen may increase transcription of the Cx-43 gene through direct mechanisms (via the putative oestrogen response elements) or indirect mechanisms (by increased expression of c-fos and c-jun acting via the putative AP-1 sites). Since oestrogen may be an important modulator of myometrial activation, these mechanisms may be critical to the processes leading to increased synthesis of gap junctions at term and, hence, to the onset of labour.