Research strongly supports a key role for calcium dyshomeostasis in Alzheimer’s disease (AD) onset and progression. This dysregulation may initiate the over production of AD-linked ‘toxic’ metabolites such as amyloid beta (Aβ) and subsequently establish a degenerative feed-forward cycle between the two entities. In this study, we have examined the role that the endoplasmic reticulum (ER) plays in both maintaining calcium homeostasis and in mediating intracellular signalling processes, with a specific emphasis on how these functions may be disrupted in AD. Cultured hippocampal neurons were prepared from control and transgenic 3xTg-AD1 mice and TgF344-AD2 rats between 3-6 days old. The magnitude of intracellular somatic calcium signals was determined by area under the curve analysis, following loading with the calcium sensitive dye, fluo-2 AM (150μM). Experiments were carried out at room temperature with neurons continuously perfused (2ml/min) with a standard saline solution (HBSS) containing TTX (1μM). A particular calcium loading protocol was adopted which involved pre-loading the ER with Ca2+ (using a brief depolarising stimulus; an extracellular application of 15mM K+) followed by application of a specific group 1 metabotropic receptor agonist (I-mGluR), (S)-3,5- dihydroxyphenylglycine (DHPG; 50µM). Such conditions are thought to crudely mimic ‘synaptic activity’ and have been previously shown to elicit so called ‘supralinear’ or enhanced calcium responses in rat hippocampal neurons3. Data, unless otherwise stated, were analysed using Wilcoxon matched-pairs signed rank test and are expressed as mean ± S.E.M In control neurons, from both murine models, I-mGluR activation combined with the loading stimulus, evoked enhanced somatic Ca2+ signals relative to I-mGluR activation alone (mouse model, 679 ± 128 %, P = 0.0013, n = 47; rat model, 6948 ± 1821 %, P = < 0.0001, n = 46). In contrast, we did not observe enhanced responses in neurons derived from TgF344-AD rats (P = 0.6084, n = 28) and, further, responses were significantly reduced in neurons derived from 3xTg AD mice (79 ± 14 %, P = 0.0006, n = 36). Notably, we observed significantly larger responses to I-mGluR activation alone, in transgenic neurons of both species compared with control neurons (mouse model, P = 0.0013, n = 36; rat model, P = 0.0044, n = 28; unpaired t test), suggesting a pathological increase in ER calcium levels. The fact that such stark alterations in calcium homeostasis and signalling have been observed in neurons from rodent models of AD at such a young age (<6 days) suggests that calcium dysregulation may occur at a much earlier stage in the disease progression than previously thought, long before any significant elevations in Aβ concentration or cognitive deficits become apparent.