Inositol trisphosphate (IP₃)-mediated Ca²⁺ release is an important physiological regulator of neuronal signaling. Conversely, pathological disruptions in intracellular Ca²⁺ signaling are implicated in neurodegenerative disorders such as Alzheimer’s disease (AD). We recently studied a transgenic AD mouse model expressing the presenilin1_M146V (PS1) mutation, and found that cortical neurons exhibited a significant enhancement of IP₃-evoked Ca²⁺ release (>3-fold) relative to age-matched non-transgenic controls. Moreover, the enhanced Ca²⁺ signals decreased neuronal excitability by increasing an IP₃-evoked hyperpolarization (Stutzmann et al., 2004). These results support the Ca²⁺ hypothesis of AD, which implicates long term Ca²⁺ dyshomeostasis as a significant contributor to AD pathology.Although the PS1 mutation is sufficient to result in human AD, it does not lead to the classical histopathological markers of AD in mice. We therefore measured IP₃ and spike-evoked neuronal Ca²⁺ signals in a novel triple transgenic (3Tg) mouse model of AD expressing mutant PS1, amyloid precursor protein and tau that does develop plaques and tangles at older ages (Oddo et al., 2003). Mice (4-5 weeks old) were deeply anesthetized with halothane (depth determined by toe pinch reflex) and rapidly decapitated. Cortical brain slices were prepared using a vibrating microslicer. By combining whole cell patch clamp recording, flash photolysis of caged IP₃ and 2-photon imaging, we determined that IP-evoked Ca²⁺ signals were significantly larger (~ 2-fold), and showed a faster rate of rise in cortical pyramidal neurons of 3Tg mice compared to age-matched controls, but were not greater than in mice expressing the PS1 mutant alone. In both transgenic models the Ca²⁺ enhancement resulted from an increase in the proportion of neurons responding to IP₃ together with a potentiation of maximal responses. Moreover, Ca²⁺ clearance rates and action potential-evoked Ca²⁺ signals were unchanged, indicating that the AD-linked mutations specifically disrupt intracellular Ca²⁺ liberation rather than affecting Ca²⁺ signaling and uptake in general. Our results implicate the PS1 mutation as a major contributor to ER Ca²⁺ dysregulation in AD, and show that this precedes the formation of plaques and tangles. It remains to be determined how dysregulated Ca²⁺ signaling interacts with other AD-linked mutations to promote the overt pathology of AD.