To determine whether the concentrations of calcium-binding proteins present in some neurons and sensory cells are sufficient to influence presynaptic calcium signaling, we studied the predominant calcium-binding protein in a class of sensory hair cells in the frog ear. Based on antibody affinity and molecular weight, we identified this protein as calretinin. We measured its cytoplasmic concentration to be approximately 1.2 mM, sufficient to bind approximately 6 mM Ca2+. Calcium signaling was altered when the diffusible cytoplasmic components were replaced by an intracellular solution lacking any fast calcium buffer, and was restored by the addition of 1.2 mM exogenous calretinin to the intracellular solution. We conclude that calretinin, when present at millimolar concentration, can serve as a diffusionally mobile calcium buffer/transporter capable of regulating calcium signaling over nanometer distances at presynaptic sites.