Mechanisms of synaptic plasticity in CNS circuits are commonly investigated using in vitro preparations such as brain slices or slice culture. During their preparation, slices are exposed to low temperatures, and electrophysiological measurements are sometimes made below physiological temperature. Because dendritic spines, which occur at the majority of excitatory synapses, are morphologically plastic, we investigated the influence of reduced temperature on their morphology and plasticity using live cell imaging of hippocampal slices from transgenic mice expressing a green fluorescent protein-based neuronal surface marker and electron microscopy of adult brain slices. Our data show that dendritic spines are highly sensitive to reduced temperature with rapid loss of actin-based motility followed at longer times by reversible loss of the entire spine structure. Thus, reduced temperature significantly affects synaptic morphology, which is in turn known to influence several key aspects of synaptic transmission. Evidence that hypothermia potentiates anesthesia and is associated with spine loss in hibernating animals further suggests that spine morphology may have a widespread influence on brain function.