The conversion of the electrically silent pregnant uterus to highly excitable at term represents a dramatic physiological event which is poorly understood. Here we provide the first description, from single-channel recordings, of a large conductance (212 pS) calcium-activated potassium channel (BKCa) in human pregnant myometrium which, in labour tissue, is either absent or has been considerably altered in its physiological and pharmacological properties. In the latter, the K+ channels have an identical conductance (221 pS) and K+ selectivity to BKCa channels but exhibit no Ca2+ or voltage sensitivity. We have termed these BK channels. Furthermore, the activity of the BKCa channel from pregnant tissue is inhibited by internal application of Ba2+ but not tetraethylammonium (TEA), whereas the activity of the BK channel is sensitive to internal TEA but not Ba2+. The role of the BKCa channel may be to suppress myometrial activity during gestation whereas BK channel activity may be important in providing a Ca(2+)-independent K+ conductance which would allow cytoplasmic Ca2+ levels to rise without activating a counteracting Ca(2+)-dependent outward current, normally provided by the BKCa channels which, by its very nature, would tend to oppose depolarization. The findings suggest that K+ channels may have an important role in determining the functional activity of the myometrium.