Potassium channels are a group of ubiquitously expressed proteins that serve numerous functions in excitable and non-excitable cells. One class of integral membrane potassium channels is the large conductance, calcium-activated potassium channel (Maxi K+). Maxi K+ differs from most other potassium channels in that its activation is controlled by both increases in intracellular calcium and by membrane depolarization. Maxi K+ dual activation is possible because of its structure. The core of the channel, which is similar to other potassium channels, is a Maxi K+ alpha homotetramer that contains both a voltage sensor and an intracellular calcium binding domain. In vascular smooth muscle, an auxiliary beta-subunit is found in a 1:1 stoichiometry. The beta-subunit exhibits its effect on the Maxi K+ channel by effectively decreasing by 5- to 10- fold the concentration of calcium required to keep the pore open. Maxi K+ beta is the target for possible therapeutics because of its role in blood flow and blood pressure regulation.