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Psilocybin (a serotonin 2A, or 5-HT 2A , receptor agonist) has shown preliminary efficacy as a treatment for mood and substance use disorders. The current report utilized positron emission tomography (PET) with the selective 5-HT 2A receptor inverse agonist radioligand [ 11 C]MDL 100,907 (a.k.a. M100,907) and cortical regions of interest (ROIs) derived from resting-state functional connectivity-based brain parcellations in 4 healthy volunteers (2 females) to determine regional occupancy/target engagement of 5-HT 2A receptors after oral administration of a psychoactive dose of psilocybin (10 mg/70 kg). Average 5-HT 2A receptor occupancy across all ROIs was 39.5% (± 10.9% SD). Three of the ROIs with greatest occupancy (between 63.12 and 74.72% occupancy) were within the default mode network (subgenual anterior cingulate and bilateral angular gyri). However, marked individual variability in regional occupancy was observed across individuals. These data support further investigation of the relationship between individual differences in the acute and enduring effects of psilocybin and the degree of regional 5-HT 2A receptor occupancy., Competing Interests: RG is on the Board of Directors of the Heffter Research Institute. YZ is employed by the company United Imaging Intelligence. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Barrett, Zhou, Carbonaro, Roberts, Smith, Griffiths and Wong.)