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[¹¹C]PK11195 is used in positron emission tomography (PET) studies for imaging brain inflammation in vivo as it binds to the peripheral-type benzodiazepine receptor (PBR) expressed by reactive glia and macrophages. However, features of the cellular reaction required to induce a positive [¹¹C]PK11195 signal are not well characterized. We performed [¹¹C]PK11195 PET and autoradiography in rats after transient focal cerebral ischemia. We determined [³H]PK11195 binding and PBR expression in brain tissue and examined the lesion with several markers. [¹¹C]PK11195 standard uptake value increased at day 4 and grew further at day 7 within the ischemic core. Accordingly, ex vivo [³H]PK11195 binding increased at day 4, and increases further at day 7. The PET signal also augmented in peripheral regions, but to a lesser extent than in the core. Binding in the region surrounding infarction was supported by [¹¹ …