Although PGF(2alpha) affects contractility of vascular smooth muscles, no studies to date have addressed the electrophysiological mechanism of this effect. The purpose of our investigation was to examine the direct effects of PGF(2alpha) on membrane potentials, Ca(2+)-activated K(+) (K(Ca)) channels, delayed rectifier K(+) (K(V)) channels, and L-type Ca(2+) channels with the patch-clamp technique in single rabbit middle cerebral arterial smooth muscle cells (SMCs). PGF(2alpha) significantly hyperpolarized membrane potentials and increased the amplitudes of total K(+) currents. PGF(2alpha) increased open-state probability but had little effect on the open and closed kinetics of K(Ca) channels. PGF(2alpha) increased the amplitudes of K(V) currents with a leftward shift of the activation and inactivation curves and a decrease in the activation time constant. PGF(2alpha) decreased the amplitudes of L-type Ca(2+) currents without any significant change in threshold or apparent reversal potentials. This study provides the first finding that the direct effects of PGF(2alpha) on middle cerebral arterial SMCs, at least in part, could attenuate vasoconstriction.