The purpose of our investigation was to examine the effects of prostaglandin F2α (PGF2α) on membrane potentials, Ca2⁢-activated K⁢ (KCa) channels, and delayed rectifier K⁢ (KV) channels using the patch-clamp technique in single rabbit middle cerebral arterial smooth muscle cells. PGF2α significantly hyperpolarized membrane potentials and increased outward whole-cell K⁢ currents. PGF2α increased open-state probability of KCa channels without the change of the open and closed kinetics. PGF2α increased the amplitudes of KV currents with a leftward shift of activation and inactivation curves and a decrease of activation time constant. Our results suggest that the activation of KCa and KV channels, at least in part, may lead to attenuate or counteract vasoconstriction by PGF2α in middle cerebral artery.