The AMP-activated protein kinase (AMPK) is a critical regulator of energy balance at both the cellular and whole-body levels. Two upstream kinases have been reported to activate AMPK in cell-free assays, i.e., the tumor suppressor LKB1 and calmodulin-dependent protein kinase kinase. However, evidence that this is physiologically relevant currently only exists for LKB1. We now report that there is a significant basal activity and phosphorylation of AMPK in LKB1-deficient cells that can be stimulated by Ca²⁺ ionophores, and studies using the CaMKK inhibitor STO-609 and isoform-specific siRNAs show that CaMKKβ is required for this effect. CaMKKβ also activates AMPK much more rapidly than CaMKKα in cell-free assays. K⁺-induced depolarization in rat cerebrocortical slices, which increases intracellular Ca²⁺ without disturbing cellular adenine nucleotide levels, activates AMPK, and this is blocked by STO-609. Our results suggest a potential Ca²⁺-dependent neuroprotective pathway involving phosphorylation and activation of AMPK by CaMKKβ.