Objectives: The potential use of garlic for prevention and treatment of different types of headaches has been suggested by several medieval literatures. Cortical spreading depression (CSD), a propagating wave of neuroglial depolarization, was established as a target for anti-migraine drugs. This study was designed to investigate the effect of garlic extract on CSD in adult rats. Methods: CSD was induced by KCl microinjection in the somatosensory cortex. The effects of five different concentrations of garlic oil (1-500 mu l/l) were tested on different characteristic features of CSD in necocortical slices. In in vivo experiments, the effects of garlic oil on electrophysiological and morphological changes induced by CSD were investigated. Results: Garlic oil in a dose-dependent manner decreased the amplitude of CSD but not its duration and velocity in neocortical brain slices. Garlic oil at concentration of 500 mu l/l reversibly reduced the amplitude of the field excitatory post-synaptic potentials and inhibited induction of long- term potentiation in the third layer of neocortical slices. In in vivo studies, systemic application of garlic oil (1 ml/l) for three consecutive days reduced the amplitude and repetition rate of CSD. Garlic oil also prevented of CSD-induced reactive astrocytosis in the neocortex. Discussion: Garlic oil suppresses CSD, likely via inhibition of synaptic plasticity, and prevents its harmful effects on astrocyte. Further studies are required to identify the exact active ingredient(s) of garlic oil that inhibit CSD and may have the potential to use in treatment of CSD-related disorders.