In the placental vasculature, where oxygenation may be an important regulator of vascular reactivity, there is a paucity of data on the expression of potassium (K) channels, important mediators of vascular smooth muscle tone. We therefore addressed the expression and function of several K channel subtypes in human placentas. The expression of K_V2.1, K_V9.3, BK_Ca, K_IR6.1 and TASK1 in chorionic plate arteries, veins and placental homogenate was assessed by RT-PCR and Western Blotting. Functional activity of K channels was assessed pharmacologically in small chorionic plate arteries and veins by wire myography using 4-aminopyridine, iberiotoxin, pinacidil and anandamide. Experiments were performed at 20%, 7% and 2% oxygen to assess the effect of oxygenation on the efficacy of K channel modulators. K_V2.1, K_V9.3, BK_Ca, K_IR6.1 and TASK1 channels were all demonstrated to be expressed at the message level. K_V2.1, BK_Ca, K_IR6.1 and TASK1 were all demonstrated at the protein level. Pharmacological manipulation of voltage-gated and ATP-sensitive channels produced the most marked modifications in vascular tone, in both arteries and veins. We conclude that K channels play an important role in controlling placental vascular function.