Parkinson's disease (PD) is a neurodegenerative condition which has a 1% prevalence in the over 60s and also affects young adults. As well as motor symptoms such as akinesia or rigidity, many patients also experience non-motor symptoms of which psychosis is the most common (Chang and Fox, 2016). Current treatments for Parkinson's disease psychosis include atypical antipsychotics such as quetiapine, clozapine and pimavanserin (a 5-HT2a inverse agonist). Pimavanserin has recently been approved in the USA as a PD psychosis treatment; it has been shown to have an overall effect on reducing hallucinations as a whole, but not on visual hallucinations specifically. Functional neuroimaging evidence confirms dysfunctional ventral visual pathway activity in PD psychosis with altered metabolism, blood flow and brain activation following visual stimulation (Chang and Fox, 2016). Outside of the ventral visual pathways, two imaging studies in PD patients with visual hallucinations have shown altered connectivity within the default mode network, a brain system implicated in many neuropsychiatric conditions, pointing to more widespread abnormalities (Chang and Fox, 2016). Structural imaging studies show some atrophy within the ventral visual pathways, but also implicates brain regions outside of visual processing areas, including parietal, frontal, and cerebellar and hippocampal regions (Ffytche et al., 2017). Moreover, even though the serotoninergic dysfunction underpinning Parkinson's disease psychosis is not fully understood, animal studies with psychedelics have pointed to the dimerisation of the 5-HT2A and mGlu2 receptors and the over recruitment of specific downstream signalling pathways. Src kinase inhibition is a potential mechanism for blocking the hallucinogenic effects of 5-HT2A receptor agonism. Src kinase inhibitor, Saracatinib, has shown to reduce the intensity of the psychedelic effect induced by psilocybin (a naturally occurring psychedelic found in psilocybe mushrooms (Byock, 2018)) and attenuate social cognition and brain changes in healthy volunteers. We will test the effects of Saracatinib on brain activity associated with visual processing using a visual processing task, known to be sensitive to 5-HT2a receptor stimulation in previous studies with psilocybin (Carter et al., 2004), and a visual recognition task (Meppelink et al., 2009) with known sensitivity to PD psychosis, both scanned using the latest implementation of multi-echo blood oxygen level dependent (BOLD) functional Magnetic Resonance Imaging (fMRI). We aim to conduct a double-blind crossover design study, looking at the effects of Saracatinib and placebo treatment on 26 patients who have PD with psychosis. Existing data shows that 10 days of dosing with Saracatinib will achieve a steady state level that is known to be well tolerated in people with Alzheimer's disease (Nygaard et al., 2015). Therefore, participants will be given an oral dose of 100mg of Saracatinib or placebo as two 50mg tablets to be taken once daily for 14 days. Participants will return to the clinic on day 14 for their final dose of Saracatinib or placebo, fMRI and EEG scans, cognitive assessments, physical examination and blood screen. The participants will then move onto the second treatment arm where they will receive a further 14 days of dosing with saracatinib or placebo depending on the group they were in for the first treatment arm. There will be a minimum 2-week washout between treatment arms to avoid potential carry over effects.