In guard cells, activation of anion channels (I(anion)) is an early event leading to stomatal closure. Activation of I(anion) has been associated with abscisic acid (ABA) and its elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). However, the dynamics of the action of [Ca(2+)](i) on I(anion) has never been established, despite its importance for understanding the mechanics of stomatal adaptation to stress. We have quantified the [Ca(2+)](i) dynamics of I(anion) in Vicia faba guard cells, measuring channel current under a voltage clamp while manipulating and recording [Ca(2+)](i) using Fura-2 fluorescence imaging. We found that I(anion) rises with [Ca(2+)](i) only at concentrations substantially above the mean resting value of 125 +/- 13 nm, yielding an apparent K(d) of 720 +/- 65 nm and a Hill coefficient consistent with the binding of three to four Ca(2+) ions to activate the channels. Approximately 30% of guard cells exhibited a baseline of I(anion) activity, but without a dependence of the current on [Ca(2+)](i). The protein phosphatase antagonist okadaic acid increased this current baseline over twofold. Additionally, okadaic acid altered the [Ca(2+)](i) sensitivity of I(anion), displacing the apparent K(d) for [Ca(2+)](i) to 573 +/- 38 nm. These findings support previous evidence for different modes of regulation for I(anion), only one of which depends on [Ca(2+)](i), and they underscore an independence of [Ca(2+)](i) from protein (de-)phosphorylation in controlling I(anion). Most importantly, our results demonstrate a significant displacement of I(anion) sensitivity to higher [Ca(2+)](i) compared with that of the guard cell K(+) channels, implying a capacity for variable dynamics between net osmotic solute uptake and loss.