The study examined the inhibitory effect of Atractylodes macrocephala (AM) on the uterine contraction during premature delivery and explored its electrophysiological mechanism by studying the effects of AM on the Ca(2+)-activated K(+) currents of pregnant human myometrial smooth muscle cells with or without the treatment with interleukin-6. Single cells were acutely isolated from pregnant human myometrial smooth muscles. Whole-cell Ca(2+)-activated K(+) currents were recorded by using an Axopatch1-D amplifier. The cells were divided into three groups: group A in which AM was added into perfusate, group B, in which interleukin-6 was added into perfusate) and group C in which AM was added into perfusate after addition of interleukin-6. IL-6 10 ng/mL inhibited BK(ca) by 36.9%+/-13.7% as compared with control (P<0.01). AM at 2 mg/mL raised BK(ca) by 36.7%+/-22.6% or 45.2%+/-13.7% with or without the treatment of IL-6, respectively (P<0.01). It is concluded that AM was able to enhance the BK(ca) of pregnant human myometrial smooth muscle cells treated or untreated with interleukin-6 and its effect on the BK(ca) IL-treated cells was stronger that its effect on BK(ca) of untreated cells. Our results suggested that AM can help to maintain the membrane potentials and the resting status of pregnant human myometrial smooth muscle cells.