There is substantial evidence that the endocannabinoid system and in particular the type 1 cannabinoid receptor (CB1R) is involved in epilepsy. We evaluated the in vivo effect of chronic administration of the anti-epileptic drugs valproate (VPA) and levetiracetam (LEV) on rat brain CB1 receptors using the positron emission tomography (PET) tracer [(18)F]MK-9470. Six Wistar rats were treated with VPA (200mg/kg) or LEV (50mg/kg) IP daily for 2 weeks. Dynamic imaging after intravenous injection of 18 MBq [(18)F]MK-9470 was performed on a FOCUS 220 microPET at baseline and after chronic treatment. Six animals were used as controls and were injected with saline, using the same protocol. Parametric images based on standardized uptake values (SUV) were generated and were spatially normalized to Paxinos space. These CB1R images were analyzed using a predefined volume of interest (VOI)-based analysis. Differences in SUV values between chronic and baseline scans in each condition (saline, VPA and LEV treatment) were calculated in each VOI. Direct binding affinity of the drugs at CB1R was assessed by competitive binding assay in Chinese hamster ovarian cells expressing human CB1R. Chronic injections of saline did not produce significant changes in global [(18)F]MK-9470 binding (p=0.43), nor in tracer binding in individual VOIs. We found a significant increase in global cerebral [(18)F]MK-9470 binding after chronic VPA administration compared to sham treated animals (+32.5%, p<0.001), as well as in tracer binding in all individual VOIs. After chronic administration of LEV, there was no significant change in global cerebral CB1R binding (+6.9%, p=0.81), nor in tracer binding in individual VOIs. As VPA does not exhibit high affinity for CB1R (displacement of [(3)H]-SR141716A 1.3+/-14.0%), such upregulation is most likely caused by an indirect effect on the endocannabinoid system. This increase in CB1R tracer binding and possibly signaling may represent a supplementary and new mechanism of VPA, but not LEV, since activation of CB1Rs has been shown to decrease excitability and excitotoxicity on-demand.